In the context of DASH (dynamic adaptive streaming over HTTP), what exactly is an MPD file?
It's format is formally specified? If so, where is it available?
What role does it play in the context for viewing encrypted media?
(An example file with descriptive comments concerning what the file's respective fields are and what they do, would be greatly appreciated, preferably in a manner that is more accessible and less terse than a formal specification).
Answer
The MPD is a description of the resources forming a streaming service, that a DASH client uses to determine which assets to request in order to perform adaptive streaming of the content.
The MPD is formally defined in ISO/IEC 23009-1, which is available electronically, currently without payment, at https://www.iso.org/standard/65274.html - click on the link on that page to download a PDF.
When content is encrypted, the MPD contains ContentProtection elements which include protection scheme-specific information describing the scheme in question and possibly some initialisation data for the scheme, allowing the player to determine whether it can present the media.