Microsoft specifies XLSB in [MS-XLSB], Excel (.xlsb) Binary File Format (the revision of 16 October 2026 was used here). Section numbers refer to it.
The package
An .xlsb file is a ZIP archive laid
out by the Open Packaging Conventions (ISO/IEC 29500-2), as in
XLSX: [Content_Types].xml
names the content type of each part, and _rels/.rels and the
_rels folders hold the relationships between parts (section 2.1). The
package relationship of type …/relationships/officeDocument points to
the one workbook part, usually xl/workbook.bin. Part names are free, but
Excel uses the same folders as XLSX with .bin instead of
.xml. Parts that hold workbook data are binary
records; the rest are XML
(drawings, themes, document properties) or opaque binary streams (images, VBA).
Part types
Section 2.1.7 lists every part type. Those stored as binary records:
| Part | Content type |
|---|---|
| Workbook (required, one) | application/vnd.ms-excel.main |
| Worksheet | application/vnd.ms-excel.worksheet |
| Chart Sheet | application/vnd.ms-excel.chartsheet |
| Dialog Sheet | application/vnd.ms-excel.dialogsheet |
| Macro Sheet | application/vnd.ms-excel.macrosheet |
| International Macro Sheet | application/vnd.ms-excel.intlmacrosheet |
| Worksheet Binary Index, Macro Sheet Binary Index | application/vnd.ms-excel.binIndexWs, …binIndexMs: where each block of rows starts, for fast loading |
| Shared Strings | application/vnd.ms-excel.sharedStrings |
| Styles | application/vnd.ms-excel.styles |
| Calculation Chain | application/vnd.ms-excel.calcChain |
| Comments | application/vnd.ms-excel.comments |
| External Link | application/vnd.ms-excel.externalLink |
| External Data Connections | application/vnd.ms-excel.connections |
| Metadata | application/vnd.ms-excel.sheetMetadata |
| PivotCache Definition, PivotCache Records, PivotTable | application/vnd.ms-excel.pivotCacheDefinition, …pivotCacheRecords, …PivotTable |
| Query Table | application/vnd.ms-excel.queryTable |
| Table, Single Cell Tables | application/vnd.ms-excel.table, …tableSingleCells |
| Revision Headers, Revision Log, User Names | application/vnd.ms-excel.revisionHeaders, …revisionLog, …userNames |
| Volatile Dependencies | application/vnd.ms-excel.volatileDependencies |
| Slicers, Slicer Cache | application/vnd.ms-excel.slicer, …slicerCache |
| Sort Map | application/vnd.ms-excel.wsSortMap |
| Attached Toolbars | application/vnd.ms-excel.attachedToolbars |
The other part types are shared with XLSX and other Office formats: ActiveX
controls and their binary data, Chart and Chart Drawing (DrawingML XML), Drawings, VML
Drawings, Images, OLE Object and OLE Package, Theme, Diagram parts, Custom XML Data
Storage and its properties, Custom XML Maps, Custom Data, Control Properties,
Custom Property, Model (the data model), Timelines and Timeline Cache
(application/vnd.ms-excel.Timeline+xml, …TimelineCache+xml),
Printer Settings, the core, extended, custom
and thumbnail file properties, digital signatures, VBA Project
(application/vnd.ms-office.vbaProject) and its signatures.
Encryption
A password-protected .xlsb is not a ZIP at all: like an encrypted XLSX
it is an OLE compound file holding an EncryptionInfo stream and the
encrypted package, as described in [MS-OFFCRYPTO] section 2.3.4 (section 2.2.11).