DougMac
You're very welcome. :)
Where you see "Token Bytes Available" or "Data Tokens Available", that's how much total Token Data you have available and ready to kick in if/when you run out of your Service Plan Data or Bonus Zone data, depending on the time of day. As an arbitrary example, if it showed 13.5GB of "Token Bytes Available" or "Data Tokens Available", the name depending on where you look, and you then purchased a 3GB Data Token, within the next few minutes you would see the "Token Bytes Available" change to 16.5GB.
Data is used in the following manner, with each successive data bank automatically kicking in when the prior one has been exhausted....
8:00AM to 2:00AM - Service Plan Data >> Token Data >> FAP
2:00AM to 8:00AM - Bonus Zone Data >> Service Plan Data >> Token Data >> FAP
FAP = Fair Access Policy, which is what your service is subject to when you've exhausted all applicable data during the given time period (8AM - 2AM or 2AM to 8AM). When you enter FAP your speed is throttled.
Regarding your last questions, yes, that's correct. Once the green part of the graph (Service Plan Data) drops to zero, the yellow part (Token Data) will then start dropping, as you'll be drawing from your Token Data. However, in this scenario (Service Plan Data being exhausted), during the hours of 2AM to 8AM any Bonus Zone data you still have will be used until exhausted, and only then would any Token Data you have kick in during that time period. And yes, only when the Token Data bank gets low would you really "need" to buy any more Token Data (if you don't want to go into FAP and be throttled), though you can buy Token Data at any time.
Lastly, on a related note, I would buy Data Tokens in the manner you mentioned, as in when you're getting close to running out of Token Data rather than buying them too far in advance, or a large amount of them. The reason I say this is that Data Tokens aren't refundable, and if you buy a bunch to have them ready, but for whatever reason you end up leaving HughesNet, it will end up having been a waste of money.