It's the Rating Evaluation it depends on how many games it has played. M for example means it has Master (Solid rating of over 300 games played)

Rating Evaluation (RE)
P = Preliminary rated computer (>= 1 opponent and >= 15 AND < 50 games)
F = Fully rated computer (>= 50 games vs 2 opponents)
R = Reference rated computer (> 150 games vs 5 opponents)
M = Master rated computer (> 300 games vs 10 opponents)


Regards
Nick