Hi Nirav,
To answer your question:
1. The list of char. in source fields are those whcih are activated for your Operating concern.
2. and 3. As said by Ajay, FKART is a fixed char and can not be changed (this char is already available for creating reports). However, if you want this you can create a custom char starting with WW (once created and activated, it will be available in scource field) and use this in derivation rule.
Regards,
ND