Mesial Occlusal

When the mesial buccal cusp of the maxillary first molar is distal to the mesial buccal cusp of the mandibular first molar.

There's my answer

The concavity that forms on the mesial aspect of maxillary first premolars is called the cuspid groove. It is the result of the first premolar having both a buccal and palatal root. The groove functions as an alignment guide for the maxillary cuspid as it descends into the mouth.

MF is short for Mesial Facial, which are surfaces on your tooth. Mesial is the surface of your tooth that is on the side closest toward the center of your mouth. Facial is the surface of your tooth that touches your lips.