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It was going to be used to implement a flat reward per upvote, but a flat reward per upvote is basically already baked into the curation reward algorithm, so it's not currently used for anything and should be removed.

The source code says:

Used to track activity rewards, updated on every post and comment

Looking at the code itself, it looks like it uh... I'm not sure exactly what it does. I ended up digging through the code trying to trace if it's impacting anything - but it might just be a value they are tracking but not yet using?

Interesting. Thanks for your help, I never thought to check the source code.

I've been starring at it all too much lately :) No problem! I'd love to know exactly what it does (if anything) as well.