Manual changes to rating (critic ratings) are overwritten by Plex when metadata is refreshed
This can be prevented by locking the field.
The list of locked fields is stored in metadata_items.user_fields. The rating (critic ratings) field is index 5, so metadata_items.user_fields is set to lockedFields=5 when locked.
If multiple fields are locked they are separated with a |. lockedFields=2|5|7.
The audience_rating field is index 30. To lock critic and audience, use lockedFields=5|30.
I was going to share the ones I know, but I found this. Here are a bunch already listed out:
Updating the PLEX database outside Plex - how? - #5 by HeartWare42
