Proportional (sort of) although the delegates are committed at the convention, not by the caucus. This isn’t really a true delegate election. The caucus is actually just a straw poll that is non-binding for the delegates.
Iowa GOP caucus rules have changed for this election. Monday night will be a secret-ballot primary in all but name, absolutely binding 27 national delegate votes to the presidential preference results.
Normal caucus business will take place after the presidential preference vote.