Word Document Formats in Office 2016 Technical Preview
The Office 2016 Technical Preview, released in March 2015, introduced format-level changes to how Word handled document compatibility — specifically in how it negotiated between new schema capabilities and documents created in earlier Word versions. This technical note covers those changes in their historical context: the compatibility mode trigger conditions, the DOCX XML schema differences visible in Office 2016 TP versus Office 2013 output, the practical effects on document exchange with earlier Word versions, and what those changes looked like during the preview period before RTM. Sections cover the compatibility mode system, schema changes, binary format interoperability, and the modern context for these files when encountered today. This note sits within the tech notes section.
The short version
Office 2016 Technical Preview produced DOCX files that used an updated internal XML namespace and introduced new formatting elements unsupported by Word 2010 and earlier. Documents saved in the TP without explicit compatibility mode enabled would open in Word 2010 in "Compatibility Mode" with some formatting degraded. The TP defaulted to Word 2016 compatibility in its format output, changing behaviour from Office 2013 which had defaulted to Word 2013 format. The practical impact on most routine documents was minimal, but documents using newer typography features, improved revision tracking, or the redesigned style engine showed visible differences when round-tripped through older Word versions.
Document compatibility mode: what it was and how it triggered
Word's compatibility mode system predates Office 2016 by many versions, but it became progressively more visible as the gap between format versions widened. The system works on two axes: the compatibility level stored inside the document's XML, and the compatibility level of the Word version opening the document.
When a document's internal format level exceeds what the opening Word version natively understands, Word activates Compatibility Mode — displayed in the title bar as [Compatibility Mode] — and applies a set of restrictions intended to preserve layout fidelity for the older format. In this mode, features introduced after the document's stated compatibility level are suppressed or substituted.
Office 2013 behaviour: Word 2013 defaulted to saving new documents with a compatibility level of Word 2013, meaning DOCX files from Word 2013 would trigger Compatibility Mode in Word 2010 and earlier. However, Word 2013 also offered a straightforward "Optimise for compatibility" option during save, which downgraded the compatibility level to a range that older versions could open without triggering the restricted mode. The default for new installations was a compatibility level that matched the installed version.
Office 2016 TP behaviour: The TP introduced a higher default compatibility level in new documents. The internal XML marking changed from targeting the Word 2013 processing model to targeting Word 2016, which introduced new namespace entries in the document's word/settings.xml component. Opened in Word 2013, these documents worked correctly. Opened in Word 2010 or earlier, they activated Compatibility Mode with a wider set of feature restrictions than Office 2013 documents had triggered.
Office 2016 TP updated the internal compatibility level embedded in word/settings.xml from w15 (Word 2013) to w16 (Word 2016). This manifested as a new <w:compat> block with different default values. The document XML namespace declarations also gained additional entries for features introduced in the 2016 format revision, including improved table rendering directives and updated revision markup attributes. The changes were backward-readable by Office 2013 but produced Compatibility Mode warnings in Office 2010 installations.
DOCX XML schema: what actually changed
A DOCX file is a ZIP archive containing XML components. The core document content lives in word/document.xml, styles in word/styles.xml, settings in word/settings.xml, and relationships in the .rels files. Examining these components between an Office 2013 output and an Office 2016 TP output shows the concrete differences.
Comparing DOCX files produced by Office 2013 and Office 2016 TP from equivalent document content showed three consistent structural differences. First, the word/settings.xml compatibility block in Office 2016 TP output included additional boolean flags for new rendering behaviours, totalling more entries than the 2013 equivalent. Second, documents using the redesigned track-changes markup (introduced in the TP) embedded revision metadata under a slightly different element structure in word/document.xml. Third, documents created with the new Office 2016 theme system stored theme references using an additional namespace prefix absent from Office 2013 output. None of these differences caused data loss on round-trip, but all three produced the "Compatibility Mode" designation in older Word versions.
Binary formats: DOC interoperability
The Office 2016 TP retained the same binary DOC format support as Office 2013 — it could open .doc files (Word 97–2003 Binary format) and save to them. Saving a document in .doc format from the TP triggered a downconversion process that stripped features introduced after the binary format's last revision (Office 2003).
The Office Open XML (OOXML) format used by .docx is specified in ISO/IEC 29500. The compatibility level system is an internal Microsoft convention layered on top of the standard — the ISO standard does not define "Word 2016 compatibility mode." This means third-party OOXML implementations (LibreOffice, Google Docs) handle the compatibility level differently: they typically ignore the internal compatibility flags and process the document content directly, which usually produces correct output but occasionally mishandles edge cases involving Microsoft-specific extension elements.
Practical implications for document exchange
During the preview period, the format changes created a specific exchange problem: documents created in the Office 2016 TP and sent to colleagues running Office 2010 appeared in Compatibility Mode with the associated feature restrictions. For organisations still running Office 2010 — which was common in enterprise environments in 2015 — this created friction during the preview period.
The most common user mistake with compatibility mode was misreading its meaning. Users who saw [Compatibility Mode] in the title bar frequently assumed the document was damaged or improperly saved. It is a format version indicator, not an error state. The document opens and its content is readable; only features beyond the compatibility level ceiling are suppressed. Attempting to "fix" the issue by re-saving in various formats often introduced actual formatting changes rather than resolving the cosmetic indicator.
The resolution in most enterprise scenarios was to explicitly save documents in "Word 97-2003 Compatible" format when exchange with Office 2010 recipients was required, or to instruct senders to use "Optimise for compatibility" in the Save As dialog. Neither was a satisfying long-term answer, but both were reliable workarounds.
Modern context
Office 2016 RTM shipped in September 2015, and the Technical Preview period closed. By 2026, the format changes introduced in the TP are the established baseline — every Office installation from 2016 onward uses and understands the format level that was novel during the preview. Office 2010 extended support ended in October 2020, removing the most common compatibility friction case.
Documents from the Office 2016 TP period are functionally indistinguishable from RTM-era documents in any current Word installation. If you encounter them in an archive, they open normally in modern Word with no Compatibility Mode indicator. The only scenario where the TP-era format designation still matters is when examining document metadata in forensic or archival contexts — the internal compatibility flags can help date document provenance to the preview period versus post-RTM.
For context on the broader Office 2016 platform changes, the Windows TwinUI technical note covers contemporaneous Windows shell changes that were being developed in parallel during the same 2015 timeframe.