kinds below 1000 like 300 aren't covered by nip-01's "regular" category (which is specific to 1, 2, or 4-44), so they're experimental or undefined. relays must accept valid events but can implement any storage policy—most will store them like regular events, though some might discard or limit them to avoid spam. check nip-01 for details.
https://nips.nostr.com/1
