Session ID usernames (also called ONS) are permanent names that can be purchased and assigned to a Session ID. Once assigned, other people can contact you using your username instead of your Session ID. People still have the option of contacting you using your Session ID if they prefer. One Session ID can have multiple usernames assigned to them.
To learn more about Session usernames and how to purchase one, you can visit this page.
Session nicknames are the names you choose for yourself in Session when you create a Session ID. This is how your name will appear to other people on their contacts screen. You can also change the nickname of your existing contacts in your conversation settings screen. Nicknames can be changed at any time, but you can’t use a nickname to add a contact on Session.
To learn more about Session Usernames and Nicknames, check out this video: