Score markers (or rehearsal markers) are a special text direction that MusiCAD 'knows' about. Any text put between brackets like [B] will get a box drawn around it.
If it is drawn using text style Music directions 3 it will be regarded as a score marker.
You can instruct MusiCAD to play from a score marker by pressing the corresponding letter <B>
You may click in the picture above to get more information.