Universal Robots E Series Original Instructions Manual page 198

23. Program Tab
Stacking
During Stacking, the robot arm moves to point A, then moves opposite the direction to search for
the next stack position. When the next stack position is found, the robot remembers it and
performs the special sequence.
In subsequent rounds, the robot starts the search from the remembered position, incremented by
the item's thickness along the direction.
Stacking is complete when the stack height is more than some defined number, or when a sensor
gives a signal.
UR5e 184 User Manual