tags property
The tags array can store a tag identifier as the first element of each subarray, plus arbitrary information afterward (always as strings).
This NIP defines "p" — meaning "pubkey", which points to a pubkey of someone that is referred to in the event —, and "e" — meaning "event", which points to the id of an event this event is quoting, replying to or referring to somehow.
Implementation
late List<List<String>> tags;