A user username is a unique, optional name that WhatsApp users can set in order to display their username instead of their phone number in the app. Usernames can be used in lieu of profile names when personalizing message content for individual users.

Usernames are an optional feature for users and businesses. If a username is adopted by a WhatsApp user, their username will be displayed instead of their phone number in the app. Business usernames are not intended for privacy, however. If you adopt a business username, it will not cause your business phone number to be hidden in the app.

WhatsApp users are limited to 1 username, but are able to change them periodically. Changing a username does not affect the user’s phone number or business-scoped user ID, and does not affect the user’s ability to communicate with other WhatsApp users or businesses on the WhatsApp Business Platform. User usernames have the same format restrictions as business usernames.

Source: META