The $M switch directive controls generation of runtime type information (RTTI). When a class is declared in the {$M+} state, or is derived from a class that was declared in the {$M+} state, the compiler generates runtime type information for properties and events that are declared in a published section. If a class is declared in the {$M+} state, and is not derived from a class that was declared in the {$M} state, published sections are not allowed in the class. Note that if a class is forward declared, the first declaration of the class must be declared with the $Mswitch. 

When the $M switch is used to declare an interface, the compiler generates runtime type information for all properties. That is, for interfaces, all members are treated as if they were published.