In the custom editor tab example, we saw how a simple Burp extension could be used to render and edit a custom message data format, within the Burp UI. For the purpose of demonstrating this capability, we used a trivial serialization format, in which user-supplied input is Base64-encoded within a request parameter value.
That example contained a rather obvious XSS vulnerability: the raw input contained within the serialized data is echoed unfiltered in the application's response. But although this type of bug might be obvious to a human, automated scanners will not (in general) identify any kinds of input-based vulnerabilities in cases where the raw input needs to be embedded within an unsupported serialization format. Since the scanner does not understand the format, it has no means of submitting its usual scan payloads in the way that is needed for the application to unpack and process the payloads and trigger any bugs. This means that in this situation, equipped only with the example of a custom editor tab extension, you would be restricted to manual testing for input-based bugs, which is a tedious and time-consuming process.
The extender API lets you tackle this problem by registering your extension as a provider of custom scanner insertion points. For each actively scanned request, Burp will call out to your extension, and ask it to provide any custom insertion points that are applicable to the request. Each insertion point that you provide is responsible for the job of constructing validly-formed requests for specific scan payloads. This lets your extension work with any data format, and embed the scanner's payloads within the request in the correct way.
Here, we can see Burp reporting the XSS vulnerability, which it has found via the custom "Base64-wrapped input" insertion point:
Here is the request that Burp made, and which was generated for Burp by our custom insertion point:

Here, via our custom message editor tab, is the literal scan payload that is embedded in the request:

So, with a few lines of extension code, we have taught Burp Scanner how to work with the unsupported serialization format. All of Burp's built-in scan checks can now place their payloads correctly into the application's requests, and bugs like this can be quickly found.
This repository includes source code for Java, Python and Ruby. It also contains a sample server (for ASP.NET and NodeJS) to execute a scan against.