Could you elaborate a bit more? The NIPs specify event kinds and their associated tags. Some even specify how to use them. But that's probably not what you're asking?
Discussion
Say I wanted to make a client that only showed pictures of food. This seems too object level to use a kind but only using a tag seems weak. Anyone could tag any image with ‘food’.
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.