My personal opinion is it is less at a detailed level of, say, rooms, and more at a general level. The classic AD&D A series 'Against the Slave Lords' modules (A1-A4) follow this pattern and are VERY linear in that you go from the first module in the series to the 2nd, etc. There's no bypassing one, no real 'branches' at all. The structure of each module attempts to insure that the PCs are shunted into the opening sequence of the next module correctly. Within each module things vary. Generally speaking they're fairly linear internally, but IIRC some of them are closer to a set of options or could even be approached almost as a random series of encounters, though again, progress to the next one requires certain things to happen. If the PCs go 'off the rails' somewhere in, say, A2, it could be pretty hard to get back on track.