Section: Annex A (Informative): Backward Compatibility Guidelines for Information Elements
Content: In order to preserve backward compatibility, the following rules should apply when adding or modifying information elements for existing messages. - No new mandatory (M) information elements should be added. - No new conditional (C) information elements should be added. - Any new IEs should be either: optional (O), having no conditions on their presence, or conditional-optional (CO), having conditions that should apply only to the sender and not to the receiver. Such conditions should be worded generally as follows: "This IE shall be sent over the xxx interface <condition>. The receiving entity need not check the IE's presence." - If any new conditions are added to a previously specified conditional (C) information element, these new conditions should apply only to the sender and not to the receiver. Such additional conditions should be worded generally as follows: "This IE shall be sent over the xxx interface <condition>. For this optional condition, the receiving entity need not check the IE's presence." Existing conditions for such conditional (C) IEs should be treated as before, and the presence of the IEs should remain conditional (C).