My understanding is that every block in the bitcoin blockchain has a coinbase transaction which contains a coinbase field. It is essentially an orphan field (placeholder) for use in claiming existing outputs. Since the coinbase transaction does not need to claim any existing outputs, it can be filled with any random data the miner chooses.
Would there be any adverse effects on Bitcoin (security or otherwise) if restrictions were placed on what the coinbase field can contain?
[Note: such a restriction may be desirable in order to prevent Bitcoin from being used as a parent coin in merged mining.]