Narrowly speaking, cross chain is a protocol that solves the transmission, transfer, and exchange of assets and functional states on two or more different chains. Narrowly defined side chain is a protocol that allows assets to be transferred from the main chain to other affiliated blockchains, and can be safely returned to the main chain from other affiliated blockchains. Broadly speaking, both side chain and cross chain are specific solutions to inter chain interoperability protocols, so we collectively refer to all side chain and cross chain protocols as cross chain interoperability protocols.