When using the DJI Focus with the Matrice 600 and Ronin-MX, the Ronin-MX can provide power and signal communication to DJI Focus motor through this cable, allowing you to adjust focus from a distance using the DJI Focus expansion module.
You need a drone license when you are flying as a commercial drone pilot, ie when you are using your drone for work and money-making purposes. This means, in most cases, you will need to sit the FAA Part 107 test.
You do not need a drone license when you are flying as a hobby and for purely recreational purposes. However, you will need to operate by strict rules. There are plans to introduce the requirement for hobbyists to pass an online aeronautical knowledge and safety test.