An earlier question (Relative clauses: “I did the best I could.”) asks about the antecedent of the relative clause, and there are two answers there:
The one (by @Man_From_India) accepted as the best answer by the question owner @Araucaria says that the fused-head NP (the best) is the antecedent, whereas the other answer says "best" itself is a noun, which @Araucaria rejects.
I agree with the fused-head analysis but do not agree that the antecedent of the relative clause is the best. I believe it's only best, the fused head itself, that is the antecedent, not the entire the best, because the entire nominal best I could is first formed and then the determines that nominal.
That said, whichever answer you choose — between the two answers and mine — all three answers agree that the relative clause (I could) is a postmodifier within an NP (the best I could).
But relative clauses can also be found in an AdjP (Adjective Phrase) or even an AdvP (Adverb Phrase), according to CaGEL* (p 1060):
Here, CaGEL analyzes the phrase (the fastest that he's ever been) in [8i] as an AdjP, not as a fused-head NP. But I wonder why it can't be analyzed as a fused-head NP with fastest being the fused head of fastest and person.
Now, CaGEL does agree with the fused-head analysis in other cases (p 1054):
In [65iii], CaGEL treats the phrase (the biggest that I've ever seen) as a fused-head NP.
If the answers to the quoted question are right in that the phrase (the best I could) is a fused-head NP, and if it's correct to analyze that [8i] has an AdjP whereas [65iii] has an fused-head NP, then how do you determine whether a phrase seemingly headed by a superlative adjective is a fused-head NP or an AdjP?
*The Cambridge Grammar of the English Language by Huddleston & Pullum
Best Answer
It's contextual. Its based off of the context used in the sentence. At many times the division of words stop and we must ask ourselves wherein lies the specific words intended meaning
Amongst all else feeling is invoked in and through sentences. Words have meaning which induces feeling in another. We could chop up words all day and give them fancy names which then give them repositories and we could go on forever....