Bandwidth / Booster Packs

I have a booster pack on my account and noticed on my bandwidth stats page that the bandwidth for the booster pack and the bandwidth for my regular plan are listed separately. Does this mean that if one of my domains (on either of the plans) exceeds that plan’s bandwidth, that the other plan or booster pack’s bandwidth won’t be used?

I guess what I’m asking is if the bandwidth from the 2 plans is combined into one big “pot” and each of my domains, regardless of the plan uses bandwidth from the same pot. I’d hate to go over my quota with one domain, and still have a booster pack with another 100GB of unused bandwitdh sitting there while I have to pay overages on the other plan.