What is the difference between the apposition of explanation and the substitute?

The student: O Shaykh, is it not possible for us to distinguish between the apposition of explanation according to the definition we have set for the apposition of explanation?

The Shaykh : Yes.
The student: Between the apposition of explanation and the substitute.

The Shaykh : The substitute.
The student: Yes, the substitute... and both options are permissible.

The Shaykh : Yes.
The student: So how do we distinguish now, since we have defined the apposition of explanation specifically?

The Shaykh : Yes.
The student: Specifically, a definition that excludes it, O Shaykh.