Come da livello di sicurezza, non ne sono sicuro, ma dalle definizioni di "extended distribution" che ho visto, sembra che il riferimento sia a una distribuzione su diversi canali e/o in diversi formati (cartaceo, digitale).

Esempi.

"Authors can sign up to Kbuuk for free, use the publishing software tools and sell their book through the online bookstore (ePub format) and iPad app. Premium membership is on offer for $19 per month or $199 per year and provides extended distribution through Kindle, iPad, Nook and Kobo. Of course, authors could publish to all of the individual channels directly, but Kbuuk is removing the complications of reformatting and time required to access all these." - http://bit.ly/1o4t6Ln

"However, if you can get past their stringent formatting requirements, you can get into their extended distribution channel that posts in your choice of Barnes & Noble, Apple iBooks, Kobo, Sony, and more (you can pick and choose all or some of the above)." - http://bit.ly/1u0zbcI

"Although CreateSpace and other companies are starting to roll out what they call “extended distribution,” this only means that your book has the possibility of getting into bookstores. It does not mean that there’s anyone working on your behalf, getting your book in front of buyers or selling your book to major accounts." - http://bit.ly/1n3s1ii

"If you’re looking to expand your book’s reach, our powerful Extended Distribution service will make your softcover black and white book available through not one, but TWO of the largest book distributors in the world: Lightning Source (Ingram Book Company’s printing division) and Baker & Taylor Textstream. This Extended Distribution Service provides even more exposure for your title, making it available to be purchased worldwide by over 10,000 libraries and over 50,000 bookstores." - http://bit.ly/ULyY0n

"What is Extended Distribution?
Normally when you used CreateSpace, your book would only be sold on Amazon and perhaps some other ecommerce outlets (like your own online store). However, Extended Distribution allowed retailers, libraries and other businesses to find and purchase your books in much larger quantities, which could lead to massive sales and increased exposure." - http://bit.ly/1u0z3Kp