If you are licensing a company they may have purchased more than one license and will want to be able to issue them internally.
Entering additional licenses on a single customer account therefore saves you from having to create a separate customer account for each authorized individual in that company.
However, they will all have access to the same documents and their accounts will all expire at the same time since they all share the same customer account.
Licensing Individual Users
A private customer may need to install the viewer on both their home and their work computers, in which case you would issue them with 2 licenses.
Each time a customer registers a license, the number of available licenses for their account is decremented by 1.
If you originally set this field to 1 when you set up the customer account, when the customer registers, the licenses field reduces to 0. As a result, the customer will not be able to register using that license again. This stops customers from sharing their license code information with others, because they can only register the number of times that they have been licensed.
If a customer requires another license at any stage (purchasing an additional use) you can of course enter a new value in this field so that they can register their license again.