ev - the event itself, encoded in binary format
cf - a free form configuration JSON
id - event id - truncated 8 bytes hash

these override any other filter
fi - full index: full event id, pubkey truncated hash, kind and created_at, enabling identifying and filtering search results to return only the event id of a match while enabling filtering by timestamp and allowing the exclusion of matches based on a user's mute list
pk - public key - truncated 8 byte hash of public key
pc - public key, created at - varint encoded (ltr encoder)

these index all events associated to a pubkey, easy to pick by timestamp
ca - created_at timestamp - varint encoded (ltr encoder)

these timestamps are not entirely reliable but a since/until filter these are sequential
fs - index that stores the timestamp when the event was received

this enables search by first-seen
ki - kind, created_at - 2 bytes kind, varint encoded created_at

kind and timestamp - to catch events by time window and kind
ta - kind, pubkey, hash of d tag (a tag value)

these are a reference used by parameterized replaceable events
te - event id - truncated 8 bytes hash

these are events that refer to another event (root, reply, etc)
tp - public key - truncated 8 bytes hash

these are references to another user
td - identifier - 8 byte truncated hash of identifier string

these are labels used with parameterized replaceable events to create a namespace.
tt - hashtag - 8 bytes hash of full hashtag

this enables fast hashtag searches
t - tag for other letters (literally the letter), 8 bytes truncated hash of value

all other tags, with a distinguishable value compactly encoded
t- - 8 bytes hash of pubkey

these are protected events that cannot be saved unless the author has authed, they can't be broadcast by the relay either
t? - 8 bytes hash of key field - 8 bytes hash of value field

this in fact enables search by other tags but this is not exposed in filter syntax
fw - fulltext search index - the whole word follows, serial is last 8 bytes

when searching, whole match has no prefix, * for contains ^ for prefix $ suffix
la - serial, value is last accessed timestamp
ac - serial, value is incremented counter of accesses

increment at each time this event is matched by other indexes in a result