-= Operator (Visual Basic)
Subtracts the value of an expression from the value of a variable or property and assigns the result to the variable or property.
The element on the left side of the -= operator can be a simple scalar variable, a property, or an element of an array. The variable or property cannot be ReadOnly (Visual Basic). The -= operator assigns the value on its right to the variable or property on its left.
The - Operator (Visual Basic) can be overloaded, which means that a class or structure can redefine its behavior when an operand has the type of that class or structure. Overloading the - operator affects the behavior of the -= operator. If your code uses -= on a class or structure that overloads -, be sure you understand its redefined behavior. For more information, see Operator Procedures (Visual Basic).