Disciplinary vs Functional in Substitution of Tasks by MSS

Dear Friends,

I've 2 queries regarding substitution:

1. In MSS Substitution functionality, there are two options : Disciplinary vs Functional.

But I am not getting what is the basic difference between these two. When to use what ?

Checked below thread but no clues:

2. Can we restrict substitution to some tasks such as I want to substitute only for Leave approval not Time sheet approval tasks ?

How to do this ?

Regards,

Niladri