Any custom variables that you put at the end of your link will show up in that field. For example:
?mycustom=mine&myother=stillmine will give you those 2 sets of variable passed back through the JVZIPN from Paypal.
By using the cvendthru, you can pass a username of a user already created and you will find that value in the IPN payload.
To learn more about writing your own integrated script please read the complete documentation about how to use JVZIPN. You can read the article at the following link: Create your own integrated script