Sets the source expression for this control.
A source expression is a C/AL expression that defines where to find the value of a control. The source expression is evaluated when updates are performed. A source expression can be as simple as the name of a table field; it can also be a complex C/AL expression.
For some controls, the source expression has a special meaning:
For a menu item, the setting of SourceExpr must evaluate to true or false. true means that the menu item will appear checked, false means that it will appear unchecked (normal).
For a control that has a, the (integer) source expression is used to select a bitmap from the list.