XRB is the formal ticker and designation under the X-type supernational currencies.
"M" prefix is a SI prefix, another way to write it would be XRB (millions). The reason I chose these SI dividers is I think currency amounts without decimals are easier to use and write. Anticipating a wider user-base and depreciation over time as keys are lost it seemed most likely people would eventually be using kxrb or plain xrb when it comes in to wide use.
The wallet defaults to Mxrb and all exchanges are listing it in millions right now.