sentence analysis

hi everybody

I'm not a student of english language but I love english language and specialy sentence analysis. I'm so happy that I found this forum.

I have a question about relative clauses in the example below (punctuation is not complete because I'm not sure about the type of the clauses)

Last night I went to a party which was unusual for me, because I don't usually go to them.

this can be somehow ambigious because the clause "which was unusual for me" can refer to the word "party" or to the idea of "going to a party". in the first case, the wh clause will be a defining adjective clause (not needing any comma) and the clause "because I don't usually go to them" is adverb clause of reason for wh clause.

but if the wh clause refers to the idea of the first clause (ie "going to a party") what do we call it? is it adverb clause or adjective clause? and is it defining or nondefining?

thank you in advance