This eliminates duplicate code. - Only maintain the code in one place!
To make sure the submodule is getting cloned too after cloning Poke_Transporter_GB, execute:
git submodule update --init --recursive
To update the submodule to a newer commit/different branch:
cd PCCS
git pull
git checkout <commit_or_branchname>
cd ..
git add PCCS
git commit
git push
The way it works is that a specific commit is tied to your Poke_Transporter_GB repository's PCCS folder.