The message editor is the module that handles composing a message — for e-mail, NetMail, sub-board posts, and feedback. It's a separate loadable module, independent of both the command shell and the mail/msgbase modules that invoke it. The same shell can be paired with different editors, and which editor you actually get is determined by your User Settings (the E option in the stock module) plus what the sysop has made available.
/? help menu. Always available; the fallback for any user/terminal.
The sysop installs additional editors as external programs (with an editor.tag marker so Synchronet knows they're message editors). See customization for setup.