Linking grounds to a specific point on an item

Okay, To link an item with a specific point you need to understand this,

<JD M2="a" M1="b" P1="xxx,xxx"
        ^      ^       ^   ^

A: The ground that's being linked to a specific point

B: The ground that will have the specific point.

"x,y": The x and y of the point in the ground.