How do Purchase Order Suggestions get generated?

[AFKGames]

I am trying to understand some functions of purchase orders to make them as useful as possible. Right now, I honestly find them near useless.

I create a PO for a vendor and location and there are 5 suggested items. If I open my item library and sort by vendor and out of stock, there are over a hundred items I can get from that vendor that HAVE STOCK ALERTS and are out of stock, why aren't they suggested? I have to do a ton of leg work to put together a PO and it honestly doesn't seem worth the hassle if the feature I expected doesn't work.