المتشابه: ما ازداد خفاء على المجمل بحيث لا يعلم المراد منه أصلا.
Mutashabih is what becomes more obscure than mujmal, so that its precise intended sense is not known at all.
The source presents this as the farthest degree of concealment. With mujmal, clarification by the speaker can determine the meaning; with mutashabih, the book emphasizes the human inability to settle the exact intended referent or mode.
This is not simply a label for every challenging Arabic word. It is a technical category represented by the particular examples identified in the textbook.