Note that if you consider specific airlines fares you should also consider airline's restrictions. And the original question was related to pure IATA rules.

For example, SU usually only allows open jaws with open segment within one country for cheaper fares or does not allow open jaws at all (not sure re AA rules).

Also it is the rule application which matters (again, usually airlines specific): for example, fares for transportation between area 2 and area 3 are different from fares between specific countries. Even same-point unrestricted fares may have different application.

It is also the combinability restrictions which matter as well: some fares are only combinable with fares within certain rule (even if with fares for same carrier) while other have no such restrictions...