HI, Marcol:
In more formal notation (if chord symbols can be considered formal notation), the chords would be marked as, for example, Cmaj or Cmin.
The use of CM or Cm as shorthand is newer. It's faster and more compact, but perhaps a little less clear in the end, when you start to get chords like Cm13#7add9 -- which, by the way, is a neat sounding chord. What Guitar Player magazine used to call a "Sharpzenflugen" chord -- any chord which can only be fingered with the help of a second person in a white tuxedo.
As uttingrh said, the chord symbols can be edited to suit your tastes.
Good luck.
David