Each origin-destination pair has a maximum permitted mileage (MPM) for eastward and westward. For example: SYD-YYZ via the Pacific Ocean, or SYD-YYZ via the Atlantic Ocean and Europe.
(I'm not certain exactly how those numbers are determined)
MPM only comes into play when booking reward tickets: if you come up with a routing that is unusual, it may exceed the MPM for that origin-destination pair. Typically, Aeroplan will allow you to exceed the MPM by 5%, but no more.
MPM can be looked up using some software such as
KVS.