A JSON-RPC Account defers signing of transactions & messages to the target Wallet over JSON-RPC. An example could be sending a transaction via a Browser Extension Wallet (e.g. MetaMask) with the window.ethereum Provider.
A JSON-RPC Account can just be initialized as an Address string. In the usage below, we are extracting the address from a Browser Extension Wallet (e.g. MetaMask) with the window.ethereum Provider via eth_requestAccounts: