Seed Point for Net Tracing
To trace a net the user must specify either a seed point (X,Y and stackup position) or a seed region (LLx,LLy,URx,URy) and a stackup position.
Assuming the seed point "lands" on a conductor this geometry will be selected and then any other geometries that touch or overlap will be added to the "net". If there are vias above or below any of the selected geometries these will be added and tracing will continue on the new layer or layers which the via connects.
RegionA region can be also used as a seed point. In this case all of the geometries that cross or lie inside of the region are used as seed points even if they are not connected together.
The most likely reason to use a region is for determining the nets that pass through a "hot spot." The size or diameter of a hot spot measurement is often several times larger than the line widths/gaps so an inspection must follow a number of potential nets to find the source of the hot spot.