In various contexts, for example the Argument request and the M response, one transmits what is essentially an arbitrary string. Often this will have been supplied by the user (for example, the -m option to the ci request). The protocol has no mechanism to specify the character set of such strings; it would be fairly safe to stick to the invariant ISO 646 character set but the existing practice is probably to just transmit whatever the user specifies, and hope that everyone involved agrees which character set is in use, or sticks to a common subset.