Ah. Copyright exists under and in accordance with the laws of each jurisdiction. American (federal) law applies in the USA and may also apply to the acts of those who are subject to American jurisdiction if done outside the USA. The USA is very keen on its own so-called "long arm" laws, and very opposed to those of other jurisdictions. Southern New York and one of the Texas courts (I think it's East Texas) expressly consider themselves competent to deal with matters of US copyright law that occur outside the USA but are subject to US jurisdiction.

UK copyright law expressly applies to authorisation abroad of infringment to occur in the UK and the issue of authorisation in the UK of infringments abroad is a known area of debate - but the infringment abroad can only be of the relevant foreign copyright law.

Now the fun starts. For instance someone assigns copyright - but US law used to have two separate terms, the first 28 years and then the rest, with different first owners in some circumstances - but instead it now has a right to terminate grants after a defined period (a different period for two works originating before and after a given break point). But that can only affect the US copyright, so if the termination privilege is exercised US goes one way, rest of the world the other.

Also, someone can assign the UK copyright (or any other) but not the rest, or the two in different directions.

English law still feels the effects of a reversionary right created under the 1911 Act, and in thoery will do for some time.

Different things ("qualifications") are necessary to make copyright arise in different jurisdictions. Different people are the first owner in different jurisdictions (US "work for hire" rules are different from UK "employment" rules).

What brings SOME sense to this jungle are the rules of international copyright treaties. In some countries (eg France) an international treaty makes French law. In the UK and USA, the treaty does not make local law and governments have to give effect to their international obligations by passing local acts. Sometimes (like the US Berne COnvention Implimentation Act) the local act expressly says that the international treaty does not make local law.

THe two main treaties are the Berne Convention, and teh Universal Copyright COnvention. It was no that long ago (1988) that the USA joined Berne. Now Berne is almost univerally acceded to, but there are still some places which paddle their own canoe.

Berne lays down minimum standards. - duration of life of author +50 years for most types of work. But the EU has gone longer, to life +70 - and in some places the last 20 years is subject to licences of right (called "domaine publique payante"). Most of the shorter terms that used to exist in differnt places have been extended.

Films are even more fun - UK until recently applied its own duration rules so a US film could go public domain for non-renewal (pre 1970-something) but still be in copyright in England or have had no US copyright at all until the US started reviving copyrights of foreign origin that had lapsed in the USA. But Beren now says that if a work goes out of copyright in its "home" jurisdiction, it goes out in other jurisdictions too - but this does not apply in England to copyrgihts that were already running when the rule came in, so pre-1957 US films remain an exotic study for the ENglish copyright lawyer.

THe simple answer is that copyright can be assigned separately (or licesed separately) for different jurisdictions, or parts of jurisdictions (yes, I could assign the copyright in London separately from that in Brighton) or differnt periods, or for differnet classes of act (sound only, or sound syncrhonised with film is a common split).

It has for decades been common for the UK copyright and teh US copyright to be separately owned - and which other places go with which of those two is not consistent.

THen you get down to collecting societites - the PRS control the performing right of thier members, but MCPS (mechanical right) are only agents of thier members. MCPS used to be able to licence the mechanical right of thier foreign affiliated societies (for example ASCAP or the Harry Fox agency) - but now it seems the US collecting societies will not grant mechanical licences at least for UK films to be made, so it may well be necessary for ANY US mechanical right to be cleared with the music US publisher.

THat's all a bit "stream of consciousness" so not sure how much clearer it makes it.....