The explanation from Synova in comments yesterday:For those who don’t know what an eArc is, it is an unproofed (if that’s the right term) advance reader copy. This means that changes might be made to the final book. It also costs more. If you want the final version as well, that’s a separate purchase.
Plus side is that you can have the book now instead of waiting, and get get reviews put on Amazon. (This is a review of the eArc of this book that I bought from the publisher….”)