A seal tag was an early anti-tampering device, used to defend against unauthorized additions to official documents in period.
A seal tag is actually quite simple; when a document is completed, the excess parchment at the bottom of the page was folded up. A hole was punched in the parchment and a seal tag (a string, a piece of finger-loop or other braid, a spare strip of parchment or a strip of leather) was threaded through the whole and secured with wax, which was then given the impression from a seal.
This marked the end of the document and made it impossible to alter without breaking the seal and removing the seal tag. However, because parchment remained below the fold, later additions could still be added; such additions would require the document to be re-sealed afterwards.