No society collects outside its own borders. Every pound earned abroad has to be fetched.
A UK writer's song is performed in Germany. The licence money is collected by the German society under German tariffs, because that is the only body with the legal apparatus to collect it there. has no office in Munich chasing radio stations. What it has is a reciprocal agreement: a bilateral deal under which the German society collects for PRS members in Germany, and PRS returns the favour for German writers in the UK.
That single fact shapes everything about foreign income. The money comes home through a chain of intermediaries, each of which charges for its part of the journey and adds a distribution cycle to the wait. An analyst who treats " earned in Germany" and "royalties received from Germany" as the same number will mis-model every catalogue with meaningful foreign earnings.
There are three routes home, and choosing between them is a real commercial decision: the society-to-society reciprocal chain, a local sub-publisher appointed in the territory, or direct membership of the foreign society. Each trades fees against speed and local policing. All three share one brutal dependency: if the work isn't registered correctly at the local society, with the right splits, there may be nothing to send home at all. Unmatched foreign money sits in the local society's black box (the pool of collected income that can't be matched to a rightsholder), and no reciprocal agreement can remit money the collector couldn't match.