To cash a check or apply for a job, normally you validate your identity by opening up your pocketbook or wallet to show the various forms of identity you carry with you. The prolog in a DITA topic is like that wallet, holding the information that other applications might need to know about your topic.
And just as in a witness protection program, where you can live somewhere else under a different identity, the same is possible for any topic that needs to be used in a different place, doing a different job under its assumed new identity. (This information is for your ears only: you can learn more about changing a topic’s default characteristics in the topicmeta episode.)
Did you know?
The world of business thrives on XML packages that contain metadata (the information needed to connect the two ends of the transaction) and a payload (the actual data being transmitted). Likewise, HTML and DITA topics have their metadata (head and prolog, respectively) and their payloads (their body elements).
Does it empower you as a writer to think that the message you are creating is a payload? When you add the metadata that helps your message get to the right person at the right time, then you have delivered a payload, albeit a non-weaponized one. If “the pen is mightier than the sword,” then the prolog guides the touché!