Possibly the only user of the Transport Layer was CompuServe's own Host-Micro Interface (HMI) API. HMI defined a number of commands that could be used to drive CIS, along with the possible responses to them, bypassing the command line interface. Since error correction was being used as a side effect of being built on B Plus, the possibility of incorrectly interpreting the commands or potentially garbled responses was basically eliminated. CIS expanded HMI to allow control of most of the batch-oriented interface, including functions for e-mail, conferences and file transfers.