The term "Complement" is not simply another word for the "post-Head string" -- post-Head strings are not always Complements. This is because the post-Head string is not always required to complete the meaning of the Head. Consider:
[NP My sister, who will be twenty next week,] has got a new job.
Here the relative clause who will be twenty next week is certainly a post-Head string, but it is not a Complement. Notice that it contributes additional but optional information about the Head sister. In this example, the post-Head string is an ADJUNCT. Like the other Adjuncts we looked at earlier, it contributes additional, optional information.
Adjuncts can occur in all the phrase types, and they may occur both before and after the Head. The following table shows examples of each type: