“Any student who attends must sign” is intelligible. If no one knows whether Zayd attended that morning, the wording has not become mujmal; the evidence about an event is missing.
Khafi can involve difficulty applying an otherwise intelligible term to a particular instance. Mushkil can involve competing lexical interpretations, while mujmal calls for further explanation of the intended meaning. These labels should not be used for every ordinary gap in factual knowledge.