Creating preview users allows administrators to view personalized and other sensitive content in preview mode. By default only the anonymous Visitor preview is available. (The Visitor preview user cannot be removed or changed.) Only Constituents in Constituent360 (not Administrators) can be selected as a preview user.
You can define up to ten (10) total preview users, not including the default Visitor. Once the limit of 10 is reached, the next preview user added deletes the oldest existing one, following the first-in, first-out rule.