Nested effects are generated in the same manner as crossed effects. Hence, the design columns generated by the following two statements are the same (but the ordering of the columns is different):
Note that nested effects are often distinguished from interaction effects by the implied randomization structure of the design. That is, they usually indicate random effects within a fixed-effects framework. The fact that random effects can be modeled directly in the RANDOM statement might make the specification of nested effects in the MODEL statement unnecessary.
See Table 19.6 in the section GLM Parameterization of Classification Variables and Effects in Chapter 19: Shared Concepts and Topics, for an example of the parameterization of a nested effect.