A master zone is a "source" zone, the files which will be deposited there serve as a starting point for the delivery. Your master can be any type of source, and can contain both a video file, but also single audio, subtitles, metadata files, still images, etc.
A capsule zone when it is intended to receive a deliverable (a PAD for example). It is the receptacle necessary for delivery to a third party (diffuser for example).
Thus, if you want to upload a file that will serve as a source for a delivery, you will need to create a master in your stock (producers, beneficiaries, distributors). You can then deliver from this master to a capsule of a recipient (broadcaster, pure player, distributor, etc.)
However, if you want to receive a file that meets your standards, you will need to create a capsule.