Filter Availability: What does First Available mean?

I don’t quite understand what First Available means vs Available when I use the Filter Availability. I noticed that when I use Available I have to use the clean up feature for things to disappear when I defer them but when I use First Available they disappear right away after I defer them.

You can try downloading the iBooks version of the OmniFocus 2 manuals in your iBooks app.

If you don’t want the iBook version, you can see the online manual at:

You can see the definition of First Available here: