As a patron, I have unlimited download credits, but have earned more by gallery posting and commenting in the forums. What can I do with the 35 dc that are just hanging out in my account? Just curious. 
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They just sit there until you have a need to use them. If you don't have a commercial use sub then you could use them to get cu items. Otherwise they will just wait and sit there in case there is a day when you can't subscribe anymore.
Makes perfect sense! Thanks for answering, Kaleena!