PetetKsfo

I can set my status to "available"  "not-available" or "not set".
"Not set" is the default setting if  I am offline.
Once I log on, the setting changes to green and shows as "available"
I can also manually set myself as "available" so even if I am not right at the computer but cooking or doing laundry or something, I can keep the "available" light on for up to 6 hours.
If I know that I have a client booked in the afternoon, or if I am not available at all for a day because I have some other plans, I can change the setting to show as "not available"  - either for a number of hours, for a day, or a week - in order to not waste your time if you are looking for something right now.
So I would say that if you see a "not available" - it means the person is not available right now, or probably not for the next few hours, but may be available later or the next day.
Peter