I found out the support bar does not actually move bit-by-bit like a health bar. They update the Support Us bar-level with a bigger jump on rare occasions. I believe the bar is mainly based on subscriptions, Supporter & Mentor , but is also just a general thermometer of the potential. The levels of the support bar are actually defined on that Support Us link, where it says "Current Subscriber Count", you can mouse-over the levels of the thermometer there. Purchases from the Marketplace support Roll20 and the artist-producer of the set! It definitely helps Roll20 to buy Sets.