The characteristics of the object
Once a player reaches level 65 in one of these (jeweller, shoemaker, tailor, weapon smith, or carver), he can visit Hel Munster in the Treechnid Forest to start the quest to become a magus in that profession (for instance a shoemaker can only become a shoemagus). A magus adds runes to an object of the corresponding profession (a shoemagus can only add runes to a shoe or belt), so as to modify the characteristics of the object.
There are also pseudo-professions, who are not listed in the profession slot: pet trainer and mount trainer.