DRM-PID question

I just received this from Cyberread.com:
------
"We have eliminated the need to store your "PID" for Mobipocket books
on CyberRead.com. Now, when you want to purchase a Mobipocket title,
you can just follow these easy steps:
   1. Purchase your Mobipocket Book at http://www.cyberread.com
   2. Click "Download." (found in My Orders section)
   3. Open the file. Your system should automatically select your
Mobipocket Reader.
   4. Your purchase will quickly download and appear in your
Mobipocket Reader."
-------

They receive my Mobi-format titles from Mobipocket, so how are they
getting around use of a PID?
Or maybe I just don't understand how PIDs work. That's very possible,
since I've always left such details up to the retailers.
Ed



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