Resource Credits (RC‘s) are your Energy to be active on the Platform. Everytime when ...

... you post, comment, follow, unfollow or vote in general it decreases your energy. But this energy regenerate daily at 20%.

Resource Credits are always a figure of Billions or more , short „B“. An Account with 0 SteemPower (SP) have Maximal 6 B of Energy. It doesn’t matter how many SP you have. The daily regeneration is always 20% or 100% in 5 days.