Good examples include:
(a) a death registration program, which you would only want to ever complete once for a tracked entity (so you would tick this option);
(b) an antenatal care (ANC) program, which a woman will be enrolled in once for every pregnancy she has (so you would leave this option un-ticked).
Note that even if you leave this box un-ticked, you can’t create two enrollments at the same time for a single tracked entity - eg you would have to ‘complete’ the first ANC enrollment for this woman before you could re-enrol her again in the same ANC program.