Read the sentence.

students who purchase parking spots are able to park in the lot closest to the entrance.

the underlined part of the sentence is a

restrictive phrase.
nonrestrictive phrase.
restrictive clause.
nonrestrictive clause.

The underlined part of the sentence who purchase parking spots is used as a restrictive clause in this sentence.
It is restrictive because it is essential and because it is not set off by commas. It is a clause because it has a finite verb.