as I understand it an apportionment is when you have rights on the common grazings and you can request to take a 'portion' of the common grazing for your own use but then in most cases you forfeit your right to the rest of that common grazings? Also if you take an apportionment for your own use you are responsible for fencing that portion off etc,
I'm sure someone will give you a clearer more legal definition later