vc-cvs-register-switches

vc-cvs-register-switches is a variable defined in `vc-cvs.el'.
Its value is

nil

Documentation:
Switches for registering a file into CVS.
A string or list of strings passed to the checkin program by
C-x v i. If nil, use the value of `vc-register-switches'.
If t, use no switches.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 21.1 of Emacs.