When searching e-books, the library catalog displays the number of users allowed to access an e-book at one time.
Here are some examples of E-book Access Models you may see in Primo/Catalog:
UA - Please note this e-book supports an unlimited number of users 1U - Please note that this e-book supports only one user at a time 2U, 3U, 4U etc. -same as 1U but with the corresponding number of users NL - Please note that this e-book has a limited number of downloads per year OA - Please note this e-book is Open Access