rxParLoader() flags the loader a MODEL owns, which rxSolve() applies for
that model's solve. These activate a loader directly, for solves that do not
go through rxSolve.rxUi() – an estimation method's internal solves, for
example, where the model being solved is assembled by the method rather than
handed to it.
Details
Only the registered loader whose name is active runs, so a package can inject
its own parameters without touching an unrelated model's par_ptr. Activate
for as short a span as possible and clear afterwards (on.exit()), because
the flag is global to the session, not attached to a model.