A mortise and marking gauge has two  sharp pins as opposed to two cutters.  It is suitable for marking out parallel lines with the grain or across end grain say in marking out a tenon but it is not suitable for marking out lines across the grain to form a shoulder as it leaves a ragged line. 

 It is also very useful for accurately marking out a single point or series of single points parallel to an edge to position screws etc.

Refer to "USING A MORTISE and CUTTING GAUGE"  for further guidance on using this tool.