https://www.x.org/releases/X11R7.7/doc/libX11/i18n/compose/e...
For example, em dash is Compose+minus+minus+minus, en dash is Compose+minus+minus+period.
It's also useful for foreign glyphs like Compose+a+a for the Nordic å, Compose+s+s for German ß (with Shift it becomes ẞ, which the new German orthography rules officially recognise!), Compose+quote+<vowel> for the various umlauts and Compose+i+period for the dotless ı (with Shift it becomes the dotted İ).