custom-local-buffer is a variable defined in `custom.el'.
Its value is

  • This variable's value is permanent if it is given a local binding.

Non-nil, in a Customization buffer, means customize a specific buffer.
If this variable is non-nil, it should be a buffer,
and it means customize the local bindings of that buffer.
This variable is a permanent local, and it normally has a local binding
in every Customization buffer.