yes, it is the default one.. but you could easily create a different form for that. to create your custom registration form, did you directly override the original one providen by joomla?

I rewrote the "profile" plugin which allows me to add extra fields, at first attempt I made it so that the "default" fields was always displayed (username, email, password and etc) and then I the Additional fields changed depending on what registration form I needed to display, but issue is that the first URL I register to one of these forms, will be the URL both forms redirect to on success/failure, this will end up causing confusion as people who sign up for Affiliate will be redirected to "reseller" form/success message when they submit. (Or if it fails)

well in this case you may need some additional code to change that redirect if the registration underway is the one belonging to an Affilaite or something like that...

A coworker asked me to ensure that we are speaking about the same thing the way your extension registration works is like this (correct me if I'm wrong): As a user, you use Joomla's OWN registration form first, and once you are registered and signed into Joomla, you can then click on the Affiliate registration link and then use YOUR affiliate registration form to signup as a affiliate?

YES, exactly. Actually, if you try to access the Affiliate registration link first (without being logged in), it will redirect you to the login page first.