Mandatory Elements and Codified Values

CAP alert messages contain certain mandatory elements. And some of those elements must contain particular, prescribed values. 

The mandatory elementals are highlighted in bold here:

DOM of a CAP message

The details of these elements and values are described in the OASIS (Organization for the Advancement of Structured Information Standards) document defining CAP version 1.2 (available on the OASIS website). As mentioned previously, those technical details are more of value to someone that needs to develop a program to author or process CAP messages than for someone issuing alerts. However, for your particular situation, it is important to know if any of the elements will trigger an alerting event and how the message is going to be processed (i.e., for triggering a siren).