Proposed Canon
Stuff is all sorts of things - Armor, weapons, items - created by Stuffamancy.
According to LIAB_Text_40, stuff may be polarized by a particular discipline (Thinkamancy is mentioned), and may be imprinted by Signamancy. The Vibrations of Grandiocosmic Strings cannot penetrate polarized stuff according to Isaac. LIAB Text 40
Stuff has several characteristics:
- Color and shape (Signamancy)
- Appearance Foolamancy
- Size, length, depth, height, Stats (aspects of the magic axis of Numbers).
- Ownership (the inanimate analogue of Loyalty).
Essentially what we call "Matter".
Some say all Stuff is Magic, that the two are the same thing.
 Real World References
The statement that Stuff and Magic are the same thing relates to the modern belief that, at a certain fundamental level, matter and energy are the same thing.