Short answer
HubSpot's KB: an A/B test serves two versions at the same URL, with half of visitors seeing each. Compare performance, then select a winner, which becomes the only live version. HubSpot lists form fields as something to test, such as asking only for email versus more information. Rerunning a variation publishes it immediately.
1. Start from a page
Landing pages in my demo portal.
2. A/B or adaptive
For up to five versions with traffic shifting automatically, see adaptive tests.