This snap allows you to snap to adjacent lower block perimeters and partitions. This snap is switched on by default but it can be useful sometimes to switch it off to avoid snapping to partitions in the block below.
When this snap is active, lines are also displayed at block level to represent any partitions and block perimeters from the block(s) below. Sometimes these lines can cause confusion, especially if a 2-D drawing has been loaded as well, and in this case it may help to switch the Snap to lower perimeters option off.