CTM was originally "CVS Through eMail", but has since changed scope to be
much more general.

CTM is now meant to be the definitive way to make and apply a delta between
two versions of a directory tree.

There are two parts to this, making the delta and applying it.  These are two
entirely different things.  CTM concentrates the computation-burden on the
generation of the deltas, as a delta very often is applied more times than
it is made.  Second CTM tries to make the minimal size delta.