Binary indexes are only used with compiled Microsoft HTML Help files (.CHM files). Because they are compiled, they are compressed and are considerably smaller than uncompiled indexes. They are useful if you are shipping multiple .CHM files that are grouped as one document set.
Binary indexes are required if you are:
Merging keywords from multiple .CHM files. During compilation, all index files are merged together as one index in compressed format. They are included in the master .CHM file.
Working with topic keywords. Topic keywords are keywords that are saved in individual HTML topic files rather than in the index file (.HHK). If your project uses topic keywords (even if used in combination with index file keywords), the index is compiled as a binary file.
Using keyword link controls in topics.
Manual sorting is not supported. Keywords are automatically sorted in alphabetical and numeric order.
Binary indexes are only supported with compiled Microsoft HTML Help (not with WebHelp, JavaHelp, or Oracle Help).
Custom fonts are not supported
Note: Binary indexes are currently only supported with Microsoft HTML Help (not with WebHelp, JavaHelp, or Oracle Help).
Tip: You can include a binary TOC with projects that use binary indexes. A binary TOC is intended to be used with large Microsoft HTML Help projects. It reduces the amount of time it takes to open topics from the Contents tab. For more information, see About binary contents files.