Oops, I got the name wrong. It is the 'Attachment' constraint, the icon just beside PlaneCoinicident constraint. I wanted to named as Placement, but changed idea, because I borrowed upstream Attacher icon for the constraint.
It can be used just like other constraints. Select two element and click the button. It fixes the relation between two parts by align the two selected element. If one of the part is fixed by Lock constraint (or indirectly locked by other Attachment constraint), the other part is considered fixed too. All fixed parts pre-calculates their placement before the solver. It works somewhat like asm4 constraint. In fact, I added this after the request from Zolko right before he starts his work on asm4. Guess he didn't want to wait for my release and went ahead anyway.