The DISPLAY NAME field is public and visible to everyone who visits the site.
You can make your DISPLAY NAME to be different from your account USERNAME. (Your account username is how you login and send/receive messages to other members. Account usernames cannot be changed.)
Your DISPLAY NAME can be anything and you may change it later if you want. For example, your DISPLAY NAME can be: a full name ('Alice Jones'); an abbreviation ('Alice J'); a first name only ('Alice'); a typical username ('ajones'); or anything else you want ('foogoo42').
Note: if you work in a security-sensitive area, you may want to use an abbreviation, username, or other pseudonym for your DISPLAY NAME.