I spent few evenings trying to understand how the the x and y work. Finally I got it. I'll try explain by an example.
If we would have two operators op1 with 150 xfy and op2 with 150 xfy, and three variables Q, W and E.
Can the order of (Q op1 W op2 E) be ((Q op1 W) op2 E)? Declaration of op2 says xfy, so the op1 must be of lower precedence. But because 150 (the op1 precedence) is not lower than 150 (the op2 precedence) this choice fails.
Can the order of (Q op1 W op2 E) be (Q op1 (W op2 E))? Declaration of op1 says xfy, so the op2 must have a lower or equal precedence compared to op1. And 150 <= 150 is true so the order is (Q op1 (W op2 E))