At the bottom of this image is the information: Credit Estimate (brown arrow below).
The original name was “Credits”. That confused many people. They thought that was the number of credits remaining in their account or for that user. So, we renamed it.
Credit Estimate gives you an idea of how much the message your are composing will “cost” you to send.
Why is it an estimate?
The actual credits a message costs could vary, especially if you use merge fields like first name. Someone named Sam will have fewer characters than someone named Gabriel. This could push a message into the next credit cost.
Mobile App, HUB, Fixes, Billing, credit