Sounds like a moderated community maybe? (NIP-72)
The tags of a nostr event are specified for certain event kinds. That is to say an event of a certain kind should only have the tags specified in the NIP.
If you want to use a custom kind (a kind which has not been specified in any NIP yet) you can use any tags you might need. There are different ranges of kind numbers specified in NIP-01. Relays will handle the custom events based on that.
I hope this clarified some confusion.