To my understanding--I have a few questions myself about the topic--an attributive clause is one that functions adjectivally. Relative clauses include relative adverbial clauses; e.g., It started to snow when I left work, which are different from attributive clauses introduced by a relative adverb:
Ex: It was last year when we met. <adjectival/attributive>
The relative clause when we met functions adjectivally. The adverb when heads the clause we met, which modifies the noun last year.
Let's test it:
Test 1: It was last year we met.
<You can omit when and the sentence remains grammatical>
Test 2: It was last year that we met.
<You can switch when with that and the sentence remains grammatical>
Test 3: Last year is when we met.
<You can omit expletive-it and add in a copular>
From Armchair Grammarian
A relative clause is occasionally introduced by a relative adverb: where, when, or why. A relative clause is a subordinate adjective clause whose referent is a noun or pronoun located within the main sentence clause. The word relative describes a word that refers or relates to another word or phrase within the sentence; this word or phrase of reference is called the antecedent, which is always a noun or pronoun located within the main sentence clause. Although the entire relative clause introduced by a relative adverb is adjectival, and functions to modify a noun or pronoun within the main sentence clause, the relative adverb itself modifies a verb within its own subordinate clause.Does that help?