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That is simply the amount of Voting Power (VP) that would have to be consumed in order to up vote every remaining post in the pool that your account has not yet voted. This metric is mostly used by our team to ensure fair allocation of votes. You will notice once you are over the FVL (1250 SP currently) that your Shadow Queue will be much lower. Each day you get ~20% VP to use which means over a 7 day period you get ~168% to use. When your ShadowQueue is over this amount it simply means not every post can be up voted before expiration.