From Xojo Documentation
Used to overload the - operator when used for subtraction, providing custom functionality.
Create an Operator_Subtract function in a class to specify the functionality of the - operator for that class.
In the function, the Self instance is the left operand and the other operand is passed as a parameter.
Suppose you have a class, Vector, that can store a vector. To define the - operator for this class, create a method of the Vector class called Operator_Subtract.
The Vector class has two integer properties, x and y.
Operator_Subtract is defined as:
// rhs stands for right-hand-side, the vector to be subtracted from Self
Var ret As New Vector // the difference between the two vectors
ret.x = Self.x - rhs.x
ret.y = Self.y - rhs.y