Hey Piotr, sorry for the delayed response. I'm doing a little speculating here, but I think it would be something along the lines of what money you've moved out of steemit compared to what money you've invested into steemit. So, if you start a fresh account and buy/import 40 SBD into the system, then you write a post and use all 40 SBD promoting it. Then, you end up with 50 SBD and you transfer that all out of the system. I think you would be correct that the earnings would be 10 SBD.

However, if you were creating content regularly and saved up 40 SBD, then promoted a post with it and got 50 SBD upon payout. I think they would argue that both the 40 SBD you earned previously and the 10 SBD from that particular post were taxable (assuming you transfer all 50 SBD out of the system at that time).

I'm also thinking about this based on my understanding of the US tax system, could be totally different for you. I'll have to dig through more of the comments here to see if this is roughly in line with what others are saying.