From Xojo Documentation
Used to overload the - operator, providing custom functionality. Operator_SubtractRight is the same as Operator_Subtract but the implicit Self instance is on the right instead of the left in the subtraction.
Create an Operator_SubtractRight function in a class to specify the functionality of the + operator for that class.
The ordinary (left) methods are always preferred; the Right version is used only if there is no legal left version.
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_SubtractRight.
The Vector class has two integer properties, x and y.
Operator_SubtractRight is defined as:
// lhs stands for left-hand-side, the vector to be added to Self
Dim ret As New Vector // the difference between the two vectors
ret.x = lhs.x + Self.x
ret.y = lhs.y + Self.y