What are people's definitions of an "L3"?

Particularly, is it an architectural classification (whether or not there's some kind of intermediary between it and L1), or a security classification (can the attacker steal? can they freeze funds? can they steal/freeze but only at high cost?)

I’ve only heard “L3” as an architectural classification - an intermediary between it and an L1

bridges on L2s are not commonly referred to as an “L3” but are scrutinized through a security classification

I’m seeing/hearing terms like “superchains” and “supernets” increasingly more often