Any type of data across any type of blockchain.
Polkadot is a network protocol that allows arbitrary data—not just tokens—to be transferred across blockchains.
This means Polkadot is a true multi-chain application environment where things like cross-chain registries and cross-chain computation are possible.
Polkadot can transfer this data across public, open, permissionless blockchains as well as private, permissioned blockchains.
This makes it possible to build applications that get permissioned data from a private blockchain and use it on a public blockchain. For instance, a school's private, permissioned academic records chain could send a proof to a degree-verification smart contract on a public chain.