By a document we understand an individual object containing marks. A mark is a perceptible feature of a document (normally something visible, e.g. a line in ink). Marks may be identified as tokens in so far as they are instances of types, and collections of marks may be identified as sequences of tokens in so far as they are instances of sequences of types. In other words, a mark is a token if, but only if, it is understood as instantiating a type1.
(Huitfeldt et al. 2008, 297)