(the red button could be put on https://exam.tools/portal/applicant/ too or instead)
If you're geolocated within a tight radius of an in-person session address (set open for walk-ins) during its time window, the red button will appear. Otherwise, it won't. After you press/click the red button, you are sent directly to that session's registration page. If you registered in advance, you can ignore it & enter PIN (geolocation could also autofill the in-person session callsign on applicant portal)?
If you're geolocated without a tight radius of an in-person session address, and there are online sessions set open for immediate/non-preregistration, a list of those will appear when you press/click a different button that appears in place of the red button - perhaps "Register for an online exam running right now".
If you disallow geolocation, there's no change in the appearance of any portal as it is now.