This file is used to provide binary delta updates for the OnlyOffice AppImage, since there is no other update mechanism on such version.
The command to create the zsync file goes as follows: appimagetool <path to .desktop file> -u "gh-release|<username>|<repo name>|latest|<name of appimage>".

Probably wise to put this in as a GitHub Action. Source: https://github.com/AppImage/AppImageSpec/blob/master/draft.md#github-releases