One of the major uses of parchment in period was for important legal doccuments (eg grants of property from the king to a subject).
How do you make sure that no-one adds extra items onto your doccument, falsifying ownership of more than you gave them?
Seal tags were one such way. The excess parchment at the bottom of a doccument that hadn't been written on is folded up. You punch a hole in the parchement and tie a seal tag (a string eg a piece of fingerloop or other braid, a spare strip of parchment or a strip of leather) through the holes securing the spare parchment. You then melt wax over this tag and press you seal into the wax. Anyone who wants to add a few lines to the bottom of the parchement will now have to break the wax to get to the parchement, and they won't be able to reseal it with your personal seal that proves it was written by you.