CLS (short for Corners Last Slot) is a sub-step of the MGLS method. In it, you orient all LL corners while simultaneously placing the last F2L slot corner.
There are six groups of cases, depending on the position and orientation of the last slot F2L corner.
C: correctly oriented in slot (7 cases - OLL cases where edge orientation is correct)
I: oriented clockwise in slot (8 cases)
Im: oriented counter-clockwise in slot (8 cases)
+: oriented clockwise in U-layer (27 cases)
-: oriented counter-clockwise in U-layer (27 cases)
O: correctly oriented in U-layer (27 cases)