The Custom Web capture option allows you to create your own “feature extractor” JSP that processes HTML content. The JSP returns in a list of possible formats during capture, and returns the specified HTML at run time (or playback).
For the Custom approach, the JSP file to be specified would be responsible for returning either a list of possible captures or a specified capture. The developer is responsible for writing the Java code that parses the HTML – although they could extend the HTML parsers that are used by UA for the standard feature extraction strategies.