Implicit enhancements in Basis Components

Recently I implemented some implicit enhancements in a function of function group SUPRN. This function group is part of the package S_PROFGEN. For this package the flag 'Package not extendable' is not set. S_PRFOGEN is part of the package BASIS. The BASIS package has the flag 'Package not extendable' set.

Now I am trying to do the same thing for function group BTCH.

This Function group is part of package SBTI. For package SBTI the flag 'Package not extendable' is not set. It is also part of the BASIS package.

When I try to implement an implict enhancement for the function BP_JOB_COPY, I get the message :

Object FUGR BTCH is part of the central basis and therefore cannot be enhanced
Exception of class CX_ENH_CENTRAL_BASE_NOT_ENHA

Can anyone explain why I can't enhance function group BTCH, but I can enhance function group SUPRN.