This is a set of adjacent controls consisting of a co-ordinate-difference pair and optionally followed by a slope control .
The controls generally operate independently but can be operated as a group by holding down the 'Ctrl' key when a snap/accept or accept is performed.
On the first co-ordinate difference control , if the 'Ctrl' key is held down during a snap/accept, the co-ordinate difference of the second co-ordinate difference control is set. If the next control is a slope control and if applicable, this is set as well. The function acts as if you performed a snap/accept or accept in each of the three controls simultaneously.
On the second co-ordinate difference control, if the 'Ctrl' key is held down during a snap/accept, if the next control is a slope control and if applicable, this control is set as well. The function acts as if you performed a snap/accept or accept in the two controls simultaneously.