With the Custom Listing setting enabled, if someone wants to book a custom trip they fill out a form requesting a date. It may not be a date that's offered on your public-facing calendar.
Since they aren't booking anything right then they can request whatever date they want to see if that works for you. If not, no worries, that's why it's a request!
From here, you can work with your customer to find a date and trip that works for both of you.
Important: Submitting a custom booking contact form does not book the trip. This is only a way to chat with prospective customers to see what they're looking for and start the dialogue with them.