What does "global" actually mean here? I'm assuming it's global to a given pants repo, or maybe pantsd executor, but maybe it means global to a machine? My best guess is that in a scenario with remote execution, the remote worker would be accumulating and persisting this cache across invocations

The globality depends on the

--named-caches-dir

option for a given repo. If the default; that's XDG_CACHE_HOME/pants/named_caches; so it's global on the machine for the current user.

of interest FYI: None of the three main remote execution projects (i.e., buildfarm, buildgrid, and buildbarn) support named caches. Stu did make a proposal last year to the REAPI Working Group about it. https://groups.google.com/g/remote-execution-apis/c/mA87-IDNpec/m/Vf--MBBmAgAJ