No matter what the pinheads at your alma mater say, there are no fixed rules. Generally the em-dash can substitute for—quite nattily too—parenthetical commas and the semi-colon.
The en-dash is visually too brief to do the same. It has more of a hanging feel, somewhat like a staccato ellipsis, and works nicely to show something has been cut off midw–
Or for number spans, like 1–10, and leading dashes for things like signatures.
–Ashley5
Remember that individual fonts, formally called typefaces, don’t often respect the literal “N” and “M” widths as they should—especially highly stylized or monospace faces.
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