It looks like you're using Internet Explorer 11 or older. This website works best with modern browsers such as the latest versions of Chrome, Firefox, Safari, and Edge. If you continue with this browser, you may see unexpected results.
1 user (1U) licence - just like with one physical copy of a library book, only one person can "borrow" (read online or download) the book at any time. If you want to read a 1U licence book and someone else is using it, then it's just like a library book being out on loan to someone else; you will have to wait for the book to be returned. Sadly, there is no option at the moment to reserve a book or be alerted when it becomes available. If you are waiting for a copy of a book to become available, you'll need to check back every so often to see the book's current status. If a book is not available, you'll see this in the search results
3 user (3U) licence - like a 1 user licence, except there are three copies available. If three people have "borrowed" the ebook, then you will have to wait until one copy is available.
Credit (NL) licence -. the digital library buys a number of ebook credits. One day's downloading or reading online = 1 credit. The number of credits available vary from publisher to publisher, but are normally 200 -400 credits per ebook. An NL licence with 200 credits would mean that 200 people can read the ebook online at the same time or one person could download the ebook 200 times.
Unlimited access (UA) - there is no limit on the number of people who can be using the ebook at the same time. This is obviously this best licence.
Note: Where possible the digital library tries to use UA licences. Some books, however, might only be available as a single user (1U) or three user (3U) licence.