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Flagging will lower rewards for a post. It also lowers reputation of the author if the one who flags has a higher reputation.

if the one who flags has a higher reputation.

Actually no.
Just like whoever upvotes your post contributes toward your REP increase,
whoever downvotes your post contributes toward your REP decrease.
It has nothing to do with the person's (who's doing the downvote) REP.
little SP = little change in rewards = little change in REP
lots of SP = big difference in reward = big change in REP

Did you read the Source or are you just going on what others have said like me?

What do you mean the Source?

What I meant by "source" was the source code. However, the other meaning of "source" would be applicable: I say that if berniesanders (who has a low reputation but a lot of steem power) flags your comment here, nothing would happen to your reputation. You say it would drop your reputation.

Well, I have my belief because I read it. My source is who wrote what I read. Your source would be who wrote what you presumably read. It is like religion when nobody wants to read the bible.