It is presented as a 'circumstance of another action' (like an adverb).

It is presented as some kind of 'substance' (like a noun).

It is presented as a 'qualifying action' (like a verb), but with a zero grammatical morpheme.

It is presented as a 'qualifying action,' functioning either as a property of some substance (like an adjective) or a circumstance of another action (like an adverb).