from AutoCAD help:
Controls how grips stretch multiline text that is centered horizontally. CENTERMT does not apply to stretching multiline text by using the ruler in the In-Place Text Editor.
0 - When you move a corner grip in centered multiline text, the center grip moves in the same direction, and the grip on the opposite side remains in place
1 - When you move a corner grip in centered multiline text, the center grip stays in place, and both sets of side grips move in the direction of the stretch