An existing association entity such as the StudentParentAssociation entity can be extended to add new elements.***
MUST NOT contain any identity properties.
MUST NOT contain entity-level documentation.
MUST contain at least one property.
MUST NOT contain properties included in the parent association unless additional context is provided.
MUST include any properties as optional single properties or optional collections.****
MUST NOT include required properties (as single properties or collections).
*** There are some known exceptions for extending associations. See Unsupported Modifications for more details. **** An exception applies to collections of simple types. Only string and decimal simple types may be included as collections.