This is one of the few situations where they overlap, and where they can be very probably interchangeably used: ------- set out intransitive verb : to start out on a course, a journey, or a career <set out across the sea> <set out early from his cabin> <the engineering course he had originally set out on> set off intransitive verb 1 : to start out on a course or a journey <set off for home> <saddled up and set off in pursuit> Merriam-Webster Unabridged Dictionary ------ Otherwise they are pretty much different. Suggest looking in a dictionary. But I'm definitely curious to know what others will have to say on this subject.
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set out gives a more determined and serious feel to the action, while set off gives a more casual and breezy feel to the action.Maybe that's not idiosyncratic. I can sense that, too, even as an ESL student.