A class of combinatorial structures is said to be ''constructible'' or ''specifiable'' when it admits a specification.
For example, the set of trees whose leaves's depth is even (respectively, odd) can be defined using the specification with two classes <math>\mathcal A_\text{even}</math> and <math>\mathcal A_\text{odd}</math>. Those classes should satisfy the equation <math>\mathcal A_\text{odd}=\mathcal Z\times \operatorname{Seq}_{\ge1}\mathcal A_\text{even}</math> and <math>\mathcal A_\text{even} = \mathcal Z\times \operatorname{Seq}\mathcal A_\text{odd}</math>.
