It's a pretty bad name, but music cards are essentially a card you can buy which has a download code on it. Some people have used "digital media", some have used "other", so it would be nice to have a proper option for them.
I don't think "digital media" is a good choice because it's a physical product - if you can download the songs without getting the physical product, it's no longer a music card, it's just a normal downloadable release. There are other reasons I don't think digital media is a good choice too: Since it's a physical product, many of the assumptions you can make about digital releases don't apply, e.g.
- A digital release has no packaging, a music card can have packaging.
- A digital release can't be purchased for mail-order, a music card can.
- A digital release can potentially have no cover art, a music card always has something which could be added to the Cover Art Archive (maybe one day we'll have a way to mark digital releases as having no cover art, so we can distinguish between releases where the cover just hasn't been added yet from releases where there is no cover art to add).
And if "digital media" isn't the right choice, that only leaves "other". "Other" isn't great because it doesn't tell us what it is, only what it isn't, and these already make up a pretty big chunk of the verified "other" releases (most of the things marked as "other" are actually just normal digital releases with the wrong format set), so I think that would justify adding it as its own format.
I've tagged 32 releases on https://musicbrainz.org/tag/format-music%20card/release already (putting it higher than 14 existing formats on https://musicbrainz.org/statistics/formats ...) and there's almost 100 listed on http://musicard.jp/ . There's also quite a few images on http://www.generasia.com/wiki/Category:Music_Card_Cover