differences 1.5a1r3577 vs 1.5-sync2.2

what is difference between zotero 1.5a1r3577 and zotero 1.5-sync2.2.
I realize that the former is the active development xpi trunk build, but what is the later ?
  • The latter is a tagged release of a public beta that has features not present in the 1.0.x branch. The trunk may, at times, be unstable. If you'd like to test upcoming features, use 1.5-sync2.2. If you'd like to assist in development, the trunk may be appropriate (note all warnings associated with the trunk builds & make more frequent backups than if you used one of the releases). If you want stability & to use a release that has been tested more completely, continue using 1.0.x.

    In any of these cases, it is prudent to backup your data.

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