There are two levels of facial expression: an emotional aspect and a facial language. Facial expression shows an emotion (and thought) of what a person feels inside. It is a non-verbal communication. On the other hand, facial expression in sign language is another level of language. It functions as a linguistic or grammatical aspect.
Facial markers in sign language contains some grammatical features, such as adjectives, adverbs and other speech forms. It also functions as a tone which can indicate a sentence type, such as a yes/no question, wh-question, a rhetorical question, a statement, etc. In a similar way, a vocal-auditory speaker uses vocal expression as a tone.
Grammatical markers in facial expression includes eye-shifting and eye-glancing, head-tilting and head-shifting, cheek-puffing, lip movement, nose-furrowing, and eyebrow-shifting. Facial signifiers can convey a wide variety of meanings.