Halliday & Matthiessen (2014: 497):
In this type it is the clause that expresses the temporal, causal or other enhancing relation (in the same way as in a dependent clause):
the house [[ × ~ (which/that) she lived in _ / where she lived ]]
I was invited to one [[ × ~ which I spent the entire time in _ ]]
Such clauses are defining relative clauses, like the elaborating ones except that here the definition is circumstantial. Enhancing embedded clauses are either (1) finite or (2) nonfinite. There are two types, (a) embedded enhancing relative clauses and (b) embedded enhancing clauses.