Because a relative noun is incomplete by itself, it is followed by material that removes the ambiguity. This completing material is called ṣilat al-mawṣūl. It may be:
Type of ṣilah
What it looks like
nominal sentence
a subject and predicate
verbal sentence
a verb with what completes it
phrase resembling a sentence
a prepositional or adverbial phrase
The relative noun and its ṣilah together function as one part of the larger sentence.