There is a discrepancy below.

Relative clause = adjective clause
1. The boy who is sitting in front of you is my cousin.
Obviously, the relative clause above is adjective clause describing the boy.

However, when I rephrase the clause into an adjective phrase.
2. The boy sitting in front of you is my cousin.
Apparently, the adjective phrase becomes a present participle clause.

As we all know, participle clause = adverbial clause, for example:
3. Seeing an accident ahead, I stopped my car.
Also plainly, the participle clause above (non-finite) is adverbial clause.

So, by conjecture, Participle clauses can also be used as adjective phrases.
Have someone discover it before?