MSP billing number or MSP practitioner number is provided to you when you begin billing through MSP. This number never changes and it stays with you forever. This will be the number that will be used as your referring practitioner number.
A Payee number can change and you could actually have several of these numbers, depending on how many locations your are billing out of. The Payee number tells MSP where to send payment for claims submitted under that particular number. When you are new to MSP this number will likely be the same number as your MSP billing number.