I've looked at the reference of APL X, but I can not find a description of the left arguments, which allows a maximum range of -12 to 12. Where did you read that there is this range of values?
Dyalog APL allows a range from -12 to 12, here's a quote from the Dyalog APL Reference:
R ← X ○ Y
Y must be numeric. X must be an integer in the range ¯ 12 ≤ X ≤ 12 R is numeric.
Perhaps you are confusing APL X and Dyalog APL.
These are the (Dyalog) operations for each value of X
(-X) ○ Y | X | X ○ Y
--------------------+----+--------------
(1-Y*2)*.5 | 0 | (1-Y*2)*.5
Arcsin Y | 1 | Sine Y
Arccos Y | 2 | Cosine Y
Arctan Y | 3 | Tangent Y
(Y+1)×((Y-1)÷Y+1)*.5| 4 | (1+Y*2)*.5
Arcsinh Y | 5 | Sinh Y
Arccosh Y | 6 | Cosh Y
Arctanh Y | 7 | Tanh Y
-8○Y | 8 | (-1+Y*2)*0.5
Y | 9 | a
+Y | 10 | |Y
b×0J1 | 11 | b
*Y×0J1 | 12 | θ
X determines which of a family of trigonometric, hyperbolic, Pythagorean and complex functions to apply to Y, from the following table.
Note that when Y is complex, a and b are used to represent its real and imaginary parts, while θ represents its phase.