We investigate the ideal semigroup and the ideal class semigroup built by the fractional ideals of an ideal system on a monoid or on a domain. We provide criteria for these semigroups to be Clifford semigroups or Boolean semigroups. In particular, we consider the case of valuation monoids (domains) and of Prüfer-like monoids (domains). By the way, we prove that a monoid (domain) is of Krull type if every locally principal ideal is finite. 2000 Mathematics Subject Classification: 13C18, 13F05, 20M12, 20M14, 20M25. 2. By [12, Proposition 4.8.2]. 3. It suffices to prove that J is r-locally principal. Let M ∈ r-max(D). If J ⊂ M , then J M = D M . If J ⊂ M , then Q M ∩ D = Q implies z / ∈ Q M , hence Q M ⊂ zD M and J M = zD M .