Yeah, suki looks at it pretty much the way I do.
A slip is a VERY short-term event, followed by greater efforts to avoid its recurrence.
A relapse could start out looking like a slip, but instead of the person stopping and redoubling his/her efforts, it segues into a WTF, for at least some length of time.
These aren't "official" definitions--so far as I know, there aren't any. It just reflects what I would call the events--and I picked it up around the rooms, so I must not be the only one who looks at it that way.