chinarut -
This applies to Projects. So, if you have a project that is setup as sequential, and you look at your actions in available mode, you will only see the first action in that project because it is the only one available (until you complete it, then the very next one is available). If your project was setup as a parallel project, then you would see all of the actions in the project.
Omni describes Available like this: "things you can actually do, because they aren't blocked by earlier actions in a sequential project".
And they describe remaining like this: "anything that's not completed".
Hope that helps!