Starting points of this article are fixed point axioms for set-bounded monotone Σ1 definable operators in the context of Kripke–Platek set theory $KP$. We analyze their relationship to other principles such as maximal iterations, bounded proper injections, and Σ1 subset-bounded separation. One of our main results states that in $KP + (V\, = \,L)$ all these principles are equivalent to Σ1 separation.