Each account enables you to add as many 'Lists' as you require:

e.g. website sign ups, bookers, enquirers - non bookers , etc.

Lists can be useful because they help segment different acquisition methods for an email address and also allow you to send emails to different lists.

Each list will only allow for one instance of a unique email address. If the user unsubscribes, then the list will tag the email and you won't send them any more emails ( this is done automatically)

The downside is that if you have one person who appears on 3 lists, then they are seen as 3 people by the system. This is the same for all email marketing systems, whether using Mailchimp, constant contact or TravelMailPro. So, you could theoretically have 500 subscribers and have them on 6 lists and this would count as 3000 subscribers ( with the corresponding cost for 3000 rather than 500 subscribers)

Some people decide this is the simplest way to manage subscribers and accept the cost. If you want to avoid being charged for having the same email on 2 different lists the way to do this is by segmentation 

How to Use one list and use Segments

1. Create custom fields e.g. VIP, Honeymoon ( help article: https://help.createsend.com/admin/subscriber-custom-fields )
2. Create segments based on custom fields ( help article: https://help.createsend.com/admin/list-segmentation ) 
3. Send emails to the segment