Both, I think, assuming it's not a fixed term apportionment, which I assume no-one would try to build on:
"Any land apportioned from the common grazings for a crofters exclusive use is deemed to form part of their croft. If they later purchase their apportionment, they become owner of that part of the croft which forms the apportionment. If the apportionment is obtained by a shareholder who is not a crofter, they will acquire a tenancy right and the apportioned land will become a croft. If they later purchase the apportionment, they will become owner of the area deemed to be a croft. Some apportionments are granted for a fixed term. If these are purchased, the land may be returned back to the common grazings, at the end of the fixed term. In these cases, the owner would become the landowner of a portion of common grazings, to which shareholders would have grazings rights."
From:
http://www.crofting.scotland.gov.uk/faq