Use a colon ( : ) to separate any subtitles. If there is an alternative dividing punctuation mark such as the question mark ( ? ) or exclamation point ( ! ), use that mark instead of the colon.
Although not explicitly mentioned, the character "-" (hyphen) should also qualify as an alternative dividing punctuation mark. I propose that we also add a mention of it to said guideline alongside an example since its commonly used as a divider and people change it to the colon very often, thinking it's not a proper way to separate the subtitle from the title (I thought this before as well).
The impulse came from how Monstercat credits their compilation and best of releases, more info can be found in https://musicbrainz.org/edit/40995015 . Multiple editors on IRC also agreed that the hyphen character (whether it's the hyphen minus, hyphen or en dash) is indeed the appropriate character for the subtitle division if that's how releases are credited, so my intention is to clarify the guideline so there will be no ambiguity in the future. Thanks a lot, much appreciated!