Comment by dragonwriter

Double hyphen is replaced in some software with an en-dash (and in those, a triple hyphen is often replaced with an em-dash), and in some with an em-dash; its usually used (other than as input to one of those pieces of software) in places where an em-dash would be appropriate, but in contexts where both an em-dash set closed and an en-dash set open might be used, it is often set open.

So, it’s not unambiguously s substitute for either is essentially its own punctuation mark used in ASCII-only environments with some influence from both the use of em-dashed and that of en-dashes in more formal environments.