Combinability applies to being able to issue multiple fares on the same ticket. You cannot issue two round-trip priceable units A-B-A and B-A-B in the same ticket with the second being within the first -- you can do two roundtrips or four one-ways. On a fare that permits side trips ($$$) you may be able to issue B-C-B where C is an airport geographically close but not co-terminal with A (think: flying BOS-ORD-PVD-ORD-BOS because BOS and PVD are 60 mi apart but not co-terminal).

Faring is super complicated; if you use ITA Matrix and want to know how it works, there's a great presentation on it. If Google ever kills off ITA Matrix, my life's work will be to write a replacement.