The T system constant is built into XLISP. T represents 'true',
as oppossed to 
(setq myvar T)                     ; set MYVAR to True
(setq myvar 'T)                    ; T and 'T both evaluate to T
(if t (print "this will print")    ; if, then, else
      (print "this won't print"))
Note: Be careful with the T value. It is possible to do a setq on T and set it to other values like NIL. Some operations will still return proper T or NIL values, but the system will be in a bad state.