Ty for responses Neil and Dan.
Dan, are you saying that if I created new features (I am not BTW), that
those new features would change the coordinate system of
the underlying dataset?
Ie: I have features with GCS WGS 84 coordinates. I have a map
with world mercator projection. I create a new feature, and
the coordinates are stored in meters in the original dataset?
So I have umpteen features stored as a pair of angles, and
one feature stored with a pair of huge numbers?
Or is it rather that the new feature will cause all the values
in the original dataset to be converted to meters, and
the coordinate system description will become Web Mercator
(which I wouldn't want since I had a reason for having the
original dataset in CGS WGS 84)