This snap allows you to snap lines to the end-point of an existing edge. A green indicator is displayed when the mouse cursor is within the snap range of an end-point:
For example, to snap the starting point of a new block perimeter to the end-point of an existing block edge, move the mouse cursor onto the background and press SHIFT to lock the drawing plane to the ground plane. Keep the SHIFT key pressed and move the mouse cursor close to the required edge marker to activate the end-point snap:
Similarly to snap the starting point of a new block partition to the end-point of an existing partition, make sure that the 'End-point' snap is switched on and move the mouse cursor close to the partition end-point to activate the snap:
See also the Adding blocks and using the measure tool tutorial.