This example shows how you can use an extension to:
When an extension registers itself as an Intruder payload provider, this will be available within the Intruder UI for the user to select as the payload source for an attack. When an extension registers itself as a payload processor, the user can create a payload processing rule and select the extension's processor as the rule's action.
When Burp calls out to a payload provider to generate a payload, it passes the base value of the payload position as a parameter. This allows you to create attacks in which a whole block of serialized data is marked as the payload position, and your extension places payloads into suitable locations within that data, and re-serializes the data to create a valid request. Hence, you can use Intruder's powerful attack engine to automatically manipulate input deep within complex data structures.
This example is artificially simple, and generates two payloads: one to identify basic XSS, and one to trigger the ficititious vulnerability that was used in the previous custom scanner check example. It then uses a custom payload processor to reconstruct the serialized data structure around the custom payload.