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Thank you very much for taking time. Thats good that it is SBDs. But it is contradict to the following article, when I read that I got confused. This guy is writing in picture number 3 and at point 4 that it is USD.
https://steemit.com/steem-help/@steempowerwhale/how-to-upvote-make-money-and-avoid-costly-mistakes-quick-and-easy-help-for-noobs

as asked above...... One more question still I have in my mind about boosters.
when people spend (let me assume 5SBD), depending on the boosting power at that time, they get upvote and their total dollars become like double , it appears nearly 10$.
what is that then? If it is 10SBD, then it means they can double their money by simply by (any) booster upvote. is that right and legal?

Booster services do not work like that they will give you a certain percentage of their vote. I have never used them myself. Just go a long and create content and have fun with it :)

I'm pretty sure it's in BSD, Steemit doesn't check the market price of BSD so it wouldn't make any sense, especially when rewards are also paid only in BSD and STEEM. The post You linked to is pretty old, back when Steem Dollar (BSD) was worth almost exactly 1$ USD. The sole purpose of BSD was/is to be worth 1$ USD so it didn't really matter back then. Currently what we see is BSD being incredibly pumped which nobody could really foresee so I believe that's why the author was saying about USD.
As for boosters - I don't know them well but basing on what I've read from one of those sites' terms of service, you get a random chance to be voted in range from 1% to 50% Voting Power or whatever the maximum they specify. So, you may get double the amount you've paid or almost nothing. It's like gambling I think.

@fiufsciak !! Thank you. Your help is really increasing my understanding.
but again in above comment Just some minutes before @sykochica explained answer..... she said whatever the price of SBD is, we will paid as 1SBD = 1USD,
so again it supports the believe that these upvote dollars are like USD for us (to be paid out) but not for the steemit plate form itself (which is around 1SBD=8USD).

If you go for 100% Power then yes. However, when you go 50/50, you get 50% in BSD (which are worth 8$ USD) and 50% in STEEM (which are BSD transferred to Steem Power at a stable rate 1$ USD - highly underpriced at the time but as I said it is meant to be worth 1$ USD). I'm quite sure that's how it works but @sykochica is more experienced than me so I think we both would be glad if she paid us a visit here :)

Okay I got another point of 50/50, But this will only work when we are upvoting.
Question is when someone else is upvoting for us (on our post or comment), what are worth of those dollars?
I assume it is 1SBD=1USD (to be paid) as @sykochica said.

No, the author of the post decides how's he gonna be paid. You choose it right before posting. Upvoting person only gives you money but it's your decision how you wanna get it.

hmmm..... Now I understood, Thank you very much.

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