When adding a new employee in the platform, you will be required to add at least their FIRST NAME and LAST NAME (MIDDLE NAME is not mandatory, but could be added as well).
This information will automate to the FULL NAME field. So, when you add information to any of those fields, the FULL NAME FIELD will automatically update.
The same will happen if you don't add any details to the DISPLAY NAME field (as it isn't a mandatory field).
By default, if this field doesn't have information, it will automatically update according to FULL NAME field, because it is the name that will reflect at the top of the profile and that is used to identify the employee across the platform. However, the DISPLAY NAME can later be changed to the employee's (or company's) preference.