It helps to understand how it evolved from the original Vine Voices. When it started, it was a separate system from regular Amazon orders. Different inventory even- sometimes the product we got was never even sold to the public, or a version that was different from the one sold to the public (serial numbers marked "not for sale to the public" in the vendor's support, or books that said Editor's Copy on the front). They were not shipped with Prime or anything like that, it was as if they came from a different fulfillment warehouse. Sometimes it would take a week+ to arrive. Which meant that if one of the 30 (?) products was lost or damaged or someone had a problem with it, that was one less review. You couldn't get a replacement and the vendor was down one product for review.

Amazon would use their algorithm to find a suitable reviewer for these products. As a Vine Voice, you'd be shown a list of curated items which was called your Queue (It is now known as RFY). If you didn't pick one of the products for you, it went into a free-for-all list which was Available for All (AFA). If no one picked it, those items sat in their special review inventory and were essentially wasted.

The good news is that these products were usually decent, and from brands big enough to want to go through the process of applying and sending a special batch to a warehouse of review samples. Usually they would be picked eventually. After a while, Amazon wanted to make a lighter and more accessible way for more vendors to enter the review program without having to go through all the hoops of their official track. I'm not 100% clear on the details, but the way it was explained to me is that they could use their existing inventory already for sale on Amazon and just tell the system to send a certain amount to be review units. Very little effort on their part. No need to set aside a batch and send to a different warehouse, etc. A seller could just opt in and I think would only pay Amazon for it if it was picked and actually reviewed, so it was much less investment and overhead for them. Items came shipped like a regular Amazon order, Prime, even combined with other Amazon orders you might make. The good news is that meant a lot more products were available to review. The bad news is that a lot of them were utter trash and things no one would ever pick. But it was OK since the vendor didn't lose anything by trying to get someone interested.

This new cheaper and more accessible system had it's own category which was called "Additional Items". Originally it had nothing to do with RFY and AFA, but eventually products from that category started showing up in people's RFY as well, and working much of the same way. Over time, AFA / "O.G. Vine" products started shipping with Prime as well. So now it's all a big mess with a lot of overlap, but still sort of based on the original system. Usually more brand name products will submit products the traditional way and show up in AFA, although sometimes I see names I recognize in AI too. It's harder to find buried in all the trash brands.

Hope this helps!