What is postdependent?

Some articles on postdependent:

Auxiliary Verbs in English - Diagnostics For Identifying Auxiliary Verbs in English
... form questions etc.) and (equivalently) they can take not as a postdependent (a dependent that follows its head) ... The following examples illustrate that the negation not can appear as a postdependent of a finite auxiliary verb, but not as a postdependent of a finite full verb a ... lead to the copula be being considered an auxiliary (it undergoes inversion and takes postdependent not, e.g ...