The value for that setting is the "main_page=xxxxxx" value that will be used if the storefront needs to take a customer to the customers_authorization page to tell them that they're not yet authorized to access something in the store.

You only need to change it if you're renaming that page in Zen Cart ... which ... is almost never needed. In fact, we probably should have hidden or not even created that switch so that it wouldn't get touched ... because it probably shouldn't be touched.

By "extension" I believe it's referring to .php ... ie: don't include ".php" at the end of the page name.

So, I ask my question again ... "why are you considering changing it?" ie: what prompted you to believe it needed changing?