When creating a test page and once selected the page to test (the control page), you may decide whether to test it against new variant(s) specifically designed for this test, or test the control page against already-existing pages.
When you test a page against a new variant, by default, the variant created will be an exact copy of the control page. You can click the Edit link to modify it as you please:
Once the A/B test starts, the traffic coming to the page that is under test will be divided in two. Both variants will use the URL of the control version and a query arg will tell our plugin which specific variant should be displayed. Thus, for example, half of the visitors will see the control version in https://example.com/some-page/?nab=0 and the half will see variant B in https://example.com/some-page/?nab=1).
Alternatively, if you want to test the previously selected page against one or more already existing pages on your website, just check “Test against an already existing page” box and select the variant pages below.
Note that in an A/B test of two existing pages on your website, the traffic coming to either of the two pages will also be divided into two. So, half of the visitors who arrive to any of the two pages (for example, https://example.com/some-page/ and https://example.com/another-page/), will only be able to see the first page and the other half will see the second one. If a visitor who’s supposed to see one page tries to visit the other one, they’ll be automatically redirected to the “correct variant.”
You may also be interested in more detailed information on how to create a Page A/B Test.