DCL is VMS's command line interface. Commands start with full-on verbs (set, copy, define, print, ...). You can use partial strings in commands as long as they are unique (e.g. "set def" instead of "set default"). Imagine being able to type "pyth" at a bash prompt and it running python.

What I think the previous posts are referring to (and which I don't remember from my DCL days) is that apparently DCL only checks the first four characters for uniqueness, so "disc", "discoduck" and "discgolf" match the "disconnect" command.