CANSEELOS is a test, so it is most often used in an IF statement. What it does is calculate a direct line from the eyes of the character (typically SRC) to the item, character, or point location... and if there is anything that blocks on that path, then you can't see.
If you can see the target, the return value is 1, and if you cannot see, the return value is 0.
When testing... keep in mind that a GM has line of sight to everything!
ON=@TargOn_Item IF !(<SRC.TARG.CANSEELOS>) SRC.SYSMESSAGE @032 That item is not within line of sight RETURN 1 ENDIF ON=@TargOn_Ground IF !(<SRC.CANSEELOS <SRC.TARGP>>) SRC.SYSMESSAGE @032 That location is not within line of sight RETURN 1 ENDIF