Halliday & Matthiessen (2014: 497-9):
Here the embedded clause contains a relative that serves as a circumstance in the clause. The clause may be either (1) finite or (2) non-finite. … If the embedded clause is finite, the relative is a WH- prepositional phrase: that is, a prepositional phrase with WH- Complement (e.g. in which) or one of its variants which ... in, that ... in, ... in:
(you’re) the one [[ × I’ve always done the most for ]]
Sometimes where or when can be used in this ‘defining relative’ function, for example:
Some may precipitate directly from sea water in areas [[ × where volcanism releases abundant silica]].
We are at a juncture in history [[ × when the options are finished]].
Here where and when are relative adverbs serving as the Head of an adverbial group.