> is this a way of letting people list their products on fedex.com, or is this a way for sellers to hold their products in a fedex warehouse, sell the items on their own website, then tell fedex where to ship it?

This is the way I understood it. Sort of like a cross between Drop-shipping and FBA but the shipper already has it, eliminating the need for the "first mile" problem (getting the package from the warehouse to the shipper). This enables a seller to do very quick delivers at low cost because they're not running to a FedEx drop off point with a bunch of shipments incrementally.

My understanding of it is the latter. FedEx provides warehouse space for retailers, and ship directly from their warehouse when an order is placed.

It's a way for a seller to allow Fedex to manage their inventory. Similar to how you can buy from a third party seller on Amazon and it will be fulfilled by Amazon.