About use of "as"

Hello everyone, I saw a sentence in a grammar book, where I don't understand what "as" refers to?

It is:

"We often prefer a relative clause rather than a prepositional phrase in non-defining relative clause with "be+preposition" or with "have" as a main verb"

Here, "As" doesn't refer to the main verb(prefer). Because It doesn't make sense at all, okay then, what does it refer to? Can "as" refer to a noun? Here it is like "as" refers to "have and be+preposition"