The logic that has the ability to fill it up?
and / or
Those that are taking things out fo the FIFO
I could imagine a system where there are "bursts" of data to be taken out of the FIFO and the burst controller needs to know that it is getting close to enpty so that it handles access a little differently.
I can also imagine systems that fill a FIFO with large bursts of things and need to know when the FIFO is almost enpty so it can cram more stuff in it before the FIFO goes fully empty.
