When menu items act as a toggle key, the on or off state is indicated by the presence or absence of a check mark next to the menu item.
The SourceExpr Property
The SourceExpr property of a menu item controls whether a check mark is displayed.
Initially, the SourceExpr property is undefined. You can define it with a valid C/AL expression that evaluates to a Boolean. The check mark appears when the value is TRUE.
This feature is used in C/SIDE. For example, on the Format menu, the Snap to Grid menu item can either be on or off. When it is on, the check mark is displayed.