The FAA recently changed the marking requirement for both hobby and commercial drone operators. Prior to the rule change, operators had to display the drone registration number anyplace that did not require tools to access. Given recent drone incursions and concerns over drones rigged with explosives, the required registration number must now be clearly visible on the outside of the aircraft. This change applies to both hobbyists operators and Part 107 licensed commercial sUAV pilots. You can find drone registration and marking requirements at FAA Dronezone.