The actual revision of the Document does not match the expected revision.
If your update conflicts with a concurrent update elsewhere, the correct solution is to read the latest revision (on the client SDK: wait for an update) and then re-attempt your update until you succeed. Consider that, since the data has changed, you may not want to submit the exact same update a second time. On the JavaScript SDK, we suggest using the mutate convenience function.
The value you provide in the If-Match header should be a revision assigned by the Sync service when interacting with an object.
revision
field, part of the object descriptor.ETag
header provided in responses.