procedure Assign(
    Source: TIdPersistent
); override;

Assign is an overridden procedure used to copy property values from the persistent object in Source to the current object instance. 

Assign compares the ClassType value in the current object to the class type for the object in Source. When the values are the same, the values in the following properties are assigned to the current object instance: 

When the ClassType values do not match, the inherited Assign method is called.