Originally Posted by
findark
Is a side trip with a new fare component or usually backtracking on an MPM fare? My memory wants to say the latter, but it doesn't really make sense...
Either way, you need a new fare component to cover the side trip.
Two common ways to need a side trip are backtracking on an MPM fare that disallows it (not all do), or stopping in the same point twice on an MPM fare component. Another one is to add extra, regional mileage to a RTW or similar fare.
So, suppose you were ticketed as BKK-TYO-OSA-TYO-SFO. That could price as BKK-SFO with a TYO-OSA side trip, assuming stopovers at TYO are allowed on the fare.