Normal < Conservative < Strict < Rigorous.
1) In Rigorous 2PL, a transaction does not release any of it's locks until after it commits.
2) In Strict 2PL, a transaction does not release any of it's exclusive lock until after it commits.
This makes R-2PL more strict than S-2PL.
3) Conservative 2PL has no such lock release restrictions.
4) And Normal 2PL is the simplest one.