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Hive is feeless monetarily, yet transactions need to be performed and transactions cost resources.
The more Hive Power You have the higher the amount of resource credits you have. Every action costs RC and they replenish just as voting mana.
resource credits (rc)

Thanks! I guess I didnt noticed as I think most of my actions are off-chain (for example playing in Splinterlands, several hours does nothing as is the game the one that scribes in the blockchain)