Autodesk AUTOCAD 2006 Command Reference Manual page 534

Nearest
Runs a line from the point you specify to the nearest object and then traces
the boundary in a counterclockwise direction.
+X
Runs a line in the positive X direction from the point you specify to the
first object encountered and then traces the boundary in a counterclockwise
direction.
-X
Runs a line in the negative X direction from the point you specify to the
first object encountered and then traces the boundary in a counterclockwise
direction.
+Y
Runs a line in the positive Y direction from the point you specify to the
first object encountered and then traces the boundary in a counterclockwise
direction.
510 | Chapter 10
ray casting
direction (+X)
ray casting
direction (-X)