The "Special Offer" variant (10684211) displays advertisements on the lock screen. These are advertisements. On the one hand, they fill the screen on the sleep screen (when the eReader is in standby) and on the other hand, they appear as a small banner on the start page. These usually contain special offers on eBooks and Kindle accessories. There are no advertisements while reading the eBook.
It is called special offers and costs less because it displays book offers and advertisements while it is on standby. The version without ads costs a bit more.
I hope this answers your question!
You can load any book (in the correct format), just take the diversions via the computer. Calibre, for example, takes care of the conversion and transfer.
If your Kindle e-reader includes Special Offers, you'll receive Sponsored Screensavers and special offers, including personalized advertising, from Amazon directly on your device.
Unfortunately, I can't tell you the exact differences yet, as the data for this article has not yet been stored. As soon as this has been done, you can compare them here: https://www.digitec.ch/de/Comparison/10684211-10124449
What I can currently tell you is that the two devices differ in the supplier :-)