procedure Assign(
    Source: TIdPersistent
); override;

Assign is an overridden method in TIdDOSAttributes used to assign property values found in the Source argument to the current object instance. 

When Source is an instance of TIdDOSAttributes, the value of the FileAttributes property in Source is stored in the corresponding property in the current object instance. When Source is any other TIdPersistent class instance, the inherited Assign method is called using Source as an argument.