We regularly have customers on waiting lists for our units, since there isnt a native waitlist function, we have the customer's email address. When a unit becomes available for them, we want to allow them to book the unit without making that unit publicly available (in the chance someone else books it). Our past system would use a link that we could send to the customer via email and who ever had access to that link could book even a blocked unit. This allows for a quick and easy email to a customer on a waitlist to book a unit without us making it visible to the public.