In VPOP3 a 'User' is an object which has an associated mailbox and settings. In different software or different situations, a User may also be called a Mailbox or an Account.
VPOP3's licensing is based on Users. You purchase a licence for a certain number of Users.
You can see how many users you have currently defined by going to the Users list in the VPOP3 settings. At the top of the page it will tell you how many users you have defined. If you don't have an Unlimited User licence, then it will also tell you the maximum number of users you are allowed to create.
The relationship between VPOP3 Users and physical objects is not fixed. In many cases, each VPOP3 User may be associated an actual person, but this is not always the case.
For instance, if you have VPOP3 Enterprise, you may have a User called Sales where all your sales-related email messages go, and then several people may access that mailbox using IMAP4. So, in this case, a VPOP3 User may have several people associated with it.
On the other hand, you may have one person who wants to receive mail about different topics and have them kept totally separate - in that case, you may have several VPOP3 Users for a single human being.
By default a User will have an email address of <their username>@<your domains>. So, if your email domain is mycompany.com and you create a User called kate, their default email address will be firstname.lastname@example.org. You can change or add email addresses using aliases or Mappings.
When deciding what Users to create in VPOP3 it can be useful to think how you want mail to be sorted out, and who you want to have access to the different messages.
A common system is to create a User for each human who will receive email, then, if you have VPOP3 Enterprise, have an extra User for any shared 'department' email addresses - such as Sales or Support.
Note - if you have VPOP3 Basic, then you cannot have shared mailboxes, so if you want to have several people receive mail for a Sales type email address, do not create a separate Sales user. Instead, you should use Mappings to redirect messages for that email address to the relevant User(s).