Pretty often there is a need to add some extra attributes to a metamodel that has already been deployed. The need for those changes usually arises from changed requirements for an application. So how to enable those changes without extending the metamodel (M2 layer) itself?

There is a special property type, AnyProperty, to handle this scenario. Whan an object type (class) has a property of type AnyProperty, an object (instance of the class) can have an arbitrary number of different properties of type AnyProperty that all share the same property_type_cd. Then how to distinguish between those properties? 

All the instances of an AnyProperty descend from the same property type of a class in M2 layer, but an application who 'owns' them can give them new names. Alternatively, it is possible to use the same name, but a distinguishing prefix directly in the value field of the property. So an AnyProperty instance 'belongs' to an application and its completely up to the application how it chooses to use, denote and present this property.

Downsides (there obviously are some, right?). First, the inheritance mechanisms built into MMX Framework (MMXMD API) obviously cannot provide full support to those properties, as the names of them are not known to M2 layer. Second, AnyProperty is defined as String in M2 so an application cannot rely on the type information if it chooses to have an instance of AnyProperty that is implemented with some other datatype (eg. an XML document or a date formatted as a string).

Anyway, this mechanism (which bears some resemblance with anyAttribute element in XML Schema) enables making run-time changes in metamodel when design-time is not an option (run-time and design-time here refer to modeling, not application design), just remember to use it with care.