What are “ad hoc” reviewers/referees for journals? What are “ad hoc” reviewers/referees for journals? Is this a different role from (regular) reviewers/referees? If yes, how are they different?

I see some people use the term ad hoc somewhere in the referee-service section of their CV, and some people don’t. Is there any difference? What is the proper practice?
Ad hoc might be used in the context of conference review where reviews are usually done by the members of a Program Committee and an extra (usually a specialized expert) reviewer is brought in for one or two papers which require careful review beyond the capability or capacity of the PC members. I haven't seen it used in the context of a journal.