Contracts are given access to the ID of the chain being operated. This
will allow contracts to check if a call was made originally on this
chain, - by mixing the chain ID into a signature for example -
effectively allowing them to prevent replay of calls performed on other
chains.

Adding the chain ID to both transaction models and to relevant calls
allows contracts downstream to check if they are being executed on the
intended chain. The cost is minimal - 1 byte in the payload - and the
benefits are cross-chain replay attack protection.