What is a backfill document?

[majonathany]

On the bottom of the readme, there are instructions for incoming documents and backfilled documents. Is there a difference? How come backfilled document instructions are much more expansive? Are these documents that must be done as a set at once, or at a later time?

Since I have documents I'd like to process asynchronously, I think I would be using the incoming document instructions more so.