user121614: The configuration parameter -blockmaxweight in Bitcoin Core allows miners to set a maximum block weight they want to mine. Oddly, the default value of this parameter is 3996000 as opposed to the SegWit limit of 4000000 WU. Why is that?

user208: It's there so that the miner can replace the coinbase transaction with their own, possibly to add more outputs.

121614: <p>The configuration parameter <code>-blockmaxweight</code> in Bitcoin Core allows miners to set a maximum block weight they want to mine. Oddly, the default value of this parameter is 3996000 as opposed to the SegWit limit of 4000000 WU. Why is that?</p>
 208: <p>It's there so that the miner can replace the coinbase transaction with their own, possibly to add more outputs.</p>
