Return to Monkey Island—more commonly known as ReMI on various Discords—is set to hit Apple’s subscription service this June. Or, more specifically, Return to Monkey Island+ will. My guess is that the + indicates the game will be playable on both Mac and i[Pad]OS.
Everybody needs more ReMI in their lives, so check it out on June 6th.
Update: The 
has bestowed upon his wisdom and informed us that the + indicates that downloadable content was included. That could mean that a) the Mac version includes the horse armor or b) as the iOS version has a sandboxed demo, the full game could be the +.
Source: The Verge