@baris So the _key is as you described, the value is the user id, and the score is the watch state?
That's correct, and the records do not exist until a user explicitly changes the setting on the category page. Until they change it the default from the ACP is used.
There are 3 possible values. https://github.com/NodeBB/NodeBB/blob/master/src/categories/watch.js#L9