Under M16, the case didn't serve much of an apparent purpose. It wasn't until M4 inherited it that its powers were revealed, containing a particle cannon within, to be used only under dire circumstances (which gameplay-wise translates to it being skill-activated whenever there's three or less dolls on the field). It's otherwise known as the "Inherited Case" in English, or the "Leftover Case" in Japanese, per the name it has as M4's MOD3 Special Equipment.
Pixiv
Twitter
Bluesky
Weibo
Lofter
Tumblr
