The text describes the different types of events that can be used in the NIP protocol. The ephemeral event is one that is sent to all clients with a matching filter but is not stored by the relay. Regular events are those with a kind between 1000 and 9999, while replaceable events have a kind between 10000 and 19999, and will replace previous events of the same kind if they have a newer timestamp. Relays may choose to support these events or not.

Link: https://github.com/nostr-protocol/nips/blob/master/16.md