The multi_chain parameter it's only available for BEP-20 and ERC-20 networks. It's also an optional parameter, meaning that you don't need to set it while making a request. It's default value is 0.
This parameter creates the same address_in on both chains (ETH/ERC20 and BSC/BEP20), allowing you to receive payments on both.
Unless used together with multi_token, this address will only be valid for the token specified (eg. USDT). If our system doesn't support the token on the 'extra' chain, it will only be created on the main chain (the one you requested). For instance, our service doesn't support DOGE over ERC20, so creating an address on /bep20/doge/ with multi_chain=1 (without multi_token also set) will not create the address on ERC20.
When setting this parameter you should make sure your wallet address (address_out) supports payments also on both chains, as the funds will be forwarded to the same address (or addresses). CryptAPI cannot reverse any payment sent through a chain unsupported by your wallet.