The backset dimension for a mortise door application can be either 2-1/2” or 2-3/4”. There are less common backset dimensions of 1-1/2 and 2” as well.
This distance refers to the distance from the edge of the door to the centerline of the lockset.
If the door is new, then the backset chosen would be closest to the center of the door.
This picture illustrates a Baldwin 2-1/2” backset mortise lock body and a door display with a Baldwin thumbpiece and knob x knob mortise locks installed.