I can’t sleep so let’s answer this:
Major = regular major chord
C-E-G
M7 = major chord with added minor (flat) 7
C-E-G-Bb
Maj7 = major chord with added major 7
C-E-G-B
They could have added the explanations of the abbreviations in the manual indeed.
On topic: The elektron manuals are very useful in my opinion, even afters years of owning their boxes, so I don’t agree that they are “completely useless”. I read them fairly often. Sometimes the terminology and word choices are a bit odd, but with the right context still understandable.