One typically specifies discrete-time linear systems simply by giving the families of maps Q, W, and E, or, in the finite dimensional case , their matrices in some basis.
For instance, equations (2.12) correspond using the standard basis for E to R and T is the function defined before.
Note that, given any linear difference equation S, we can think of this as a discrete-time linear system with no controls, simply by letting D be the trivial vector space F and G.