Callisto is NOT a testnet or a hardfork of Ethereum Classic.
This indicates that changes are made to the blockchain, or will be made in the future, different from the original.
This phrase says that Callisto did not keep the balance sheets of the holders of internal network tokens. Fork or hard fork changes the software, and balances are saved.
I somewhere read that Callisto is a side-chain ETC. Does this mean that payments between these networks will be possible?
Blockchains 3.0 aim interchain operability. However, these networks are interoperable with their own copies in most general cases. We aim to research and develop a reference implementation of legacy blockchain modification which will allow for interoperability with newer blockchain projects.
We are considering projects such as EOSio, AION, and ICX. At the moment, ICX seems to be the most advanced in this field.