When you examine the unit you start to get an idea of how much thought went into designing it.
You get a range of presets for the depth AND width of the mortise - with four thicknesses from the four router bit looking cutters. The following illustration is at 1 to 1 scale so you can see the actual range of options. And sometimes having a little side to side slop can be useful.bb As you can see, it won't do through mortises in thick stock and you probably wouldn't want to do the legs for a work bench with it. But for a lot of Arts & Crafts Stickley / Greene & Greene stuff with all those mortise and tenon joints - this thing is a dream come true. We'll get to "the distance from your reference face to the centerline of the mortise" on the next page.
Doing loose tenon mortise and tenon joints that fit properly and locate the parts where you want them is all about distances from Reference Edges / Ends and Reference Faces. The built in retractable stop pins and the easy to see and read "viewing window" lets you position the center of the mortise from either the left or right Reference Edge / End . The pins can also use the either end of an already cut mortise to space the next mortise in a part. That last built in capability comes in real handy as you will see.
Once you've cut a mortise, you can use it and one of these stops to space to the next mortise. If you use the "left" end of the previous mortise you get a different spacing then if you use the "right" end of the previous mortise
For wider spacing there's the Cross Stops accessory - with precision positionable retractable stop pins - on both sides of the mortise's centerline. The notches on the fixed part and the notches on the moveable stop makes setting both left and right stops the same distance from the centerline of the mortise to be cut. AND - if you write down the numeric location - you can reset them any time you need to (or am I the only one that "occassionaly" has to go back and reproduce a part due my "screw up" / "challenge")