"Sub season" is a new term I invented just for microstock. Think about the rainy season - it sucks, and you get a lot of rain day after day when it comes. The "sub season" is just like that - it comes when you get a lot of sub. sales (but few, or no credit sale), day after day. It sucks because you don't really get much revenue, plus it lows your RPD.
Since the beginning of November, I was, unfortunately, under the sub season. I had 20 sales for the month so far, 17 of them are sub sales, only 3 credit sales. My RPD for the month was a pathetic $0.70. I felt a bit depressed and I must be in a bad luck.
And just a moment ago, here comes my biggest sale of all time (so far) - a 26-credit sale with earning of $13.5. Thanks for whoever bought this image, because you just declared that "sub season" is finally over: