First of all the pamadm modules have nothing to do with Fortran modules. With the pamadm modules it is possible to enable/disable the compilation and testing of entire directories. A pamadm module will not only select a directory to compile or ignore, but also define CPP definitions (MOD_FOO), and select tests. Compilation depends on the selected pamadm modules (pamadm generates list of sources to compile and sets compile definitions) It is important to realize that when we release or export the code with “make release”, then the result of it depends on the pamadm flags. Code that is then not activated does not make it to the release tarball. Same for tests.
It can be useful to introduce a new pamadm module for your new functionality if you want to have the functionality on master but you want to be able to disable its release. Note that this can be achieved also with using MOD_UNRELEASED.
Personal opinion (Radovan): pamadm modules are an excuse to not use branches properly. If we used feature branches properly and only merged to-be-released code to master, we would not need pamadm modules.