The username is the one you use to login to the board with. The displayed name is the one which shows alongside your posts. The displayed name and the username are, by default, the same.
If you require a change to your username you will need to ask for this to be done for you and we ask that you have a good reason. That means that the request shouldn't just be frivolous - we don't want to be inundated with frequent requests for changes depending on the state of the moon, condition of the market, closeness to holidays, or other such trivialities.
If a member has a good reason for wanting to change their username or displayed name then just ask for it to be done. It would be helpful if, along with the request, you briefly explain why the change is needed, as that will help expedite things.
Valid reasons would be to synchronise with a business name or an ID elsewhere, or to reflect a member's area of interest, etc.
Hope this helps.