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EDIT Btw: do I remeber this right, that alle the denomination fees go to the block rewards?
yep, all fees (except proposal preparation fee which is destroyed) go to block reward
Why is the proposal preparation fee destroyed? I imagine there's a good reason for it, but couldn't it go towards the block reward too?
The security model is you destroy 5 DASH via an OP_RETURN, with a hash of the proposal included in the output. This means the 5DASH is not spendable and will be excluded from the mempool afterward, also that the user can recreate the 5DASH in the proposal if it's approved. The idea really is that you should be sure enough about the proposal getting approved that you won't actually lose the 5DASH, you're just recreating it.