System transaction

The consensus engine may invoke system contracts, such transactions are called system transactions. System transactions is signed by the validator who is producing the block. For the witness node, will generate the system transactions (without signature) according to its intrinsic logic and compare them with the system transactions in the block before applying them
Enforce backoff
In Clique consensus protocol, out-of-turn validators have to wait a randomized amount of time before sealing the block. It is implemented in the client-side node software and works with the assumption that validators would run the canonical version. However, given that validators would be economically incentivized to seal blocks as soon as possible, it would be possible that the validators would run a modified version of the node software to ignore such a delay. To prevent validator rushing to seal a block, every out-turn validator will get a specified time slot to seal the block. Any block with an earlier blocking time produced by an out-turn validator will be discarded by other witness node.