وإما مجمل وهو ما لم يذكر وجهه
The other type is condensed: the common respect is not stated.
“Zayd is like a lion” gives the two terms and the comparison, but it does not explicitly say “in courage.” The audience may still infer the intended quality from context. The simile is mujmal even if its intended meaning is immediately apparent.
Thus mujmal does not mean incomprehensible. Omission is the criterion. Some unstated respects are obvious, while others require reflection or refined literary perception; that is a further distinction.