I want to know more about these.

It says the change from the last upload.

But since the uploads are at random times, how do I know when the last upload was? Maybe I missed an upload? What happens when a merchant uploads twice in a small amount of time, say they made a mistake with some of the products and needed to fix them. Will the delta be a change from the last correct datafeed or from the incorrect feed?

I might as well upload the whole thing at least now I know I have 'complete' data.

How are dropped products handed?

Some datafeeds are so huge (i.e. macy's) that download the category feeds instead, how come they don't have deltas?

Now that it has come to my attention, I think this issue has to be explored to see if this is actually a useful service or not.