A seal is a thing with a saved spell (mostly on paper). It was created so that mages could use the spell even if they ran out of mana or people with poor mana regeneration.
To use a seal, you simply break the item in which the spell is stored.
The seal Cory used was the Mana Leak Seal. It's a spell that Cory created himself.
Mana leak is a spell designed to track. Because it works like this:
All living things have mana vessels. And if there is a wound in the vessels, mana leaks out of the body and mana leakage occurs. But each creature's mana is different.
The mana leak spell, first of all, creates a mana leak. And secondly, the caster's mana starts reacting to the mana of the creature to which the spell was cast.