The edge snap allows you to snap to any point on the edge of an existing object. The edge snap can be used in conjunction with the increment snap to enable a point to be located along an edge by moving it in defined increments. A red indicator is displayed when the mouse cursor is within the snap range of an edge and distance indicators are also displayed to help in locating a point a specific distance from either end of the edge:
By switching the increment snap on, the point can be moved along the edge in defined increments. For example, if you wanted to start a block perimeter line at 6.0 metres from the end of the block, after switching on the increment snap, you could set the snap increment to say 0.5m and then move the mouse cursor to snap to the edge at a distance of 6.0 metres from the end-point:
If you wanted to locate the beginning of the perimeter line at a specific distance from the other end of the edge, you can switch the end of the edge from which the increment snap operates by toggling the 'Switch snap increment direction' switch:
See also the Adding blocks and using the measure tool tutorial.