As I said, designed to be lossless.This is just "Mastered for iTunes" rebranded. Details:
When you convert audio to a lossy codec, there are often times the signal can get distortion or digital clipping due to the lossy conversion process. ADM is just a program whereby engineers can use Apple-supplied tools to convert (and test) their audio to make sure it doesn't have any clipping when converted to lossy AAC. Basically, its just a quality-control program for 256kbps audio conversion.
EDIT: Here's the official link (note the url) https://www.apple.com/itunes/mastered-for-itunes/
The page even emphasizes this in the opening paragraph.
“The results, both for streaming and download, are virtually indistinguishable from the original 24 bit studio masters.”
That PDF supplied on that page is a great resource to elaborate and explain on it.