Sub-Space is a parallel version of Sub-Con in the Super Mario universe. A potion can create a door where characters enter, and specific items and events, such as coins and Super Mushrooms, can be obtained. In BS Super Mario USA, entering a vase while in Sub-Space will lead directly to a showdown with Wart.
Ramona Flowers takes advantage of Sub-Space in the Scott Pilgrim stories, using it to move faster around Canada as an Amazon.ca delivery girl (although it is heavily implied she used this during her time in the US as well). As the Scott Pilgrim series features many references to video games, this is a deliberate nod to Super Mario Bros. 2, and SMB2 is even mentioned in the book's dialogue as a parallel. Sub-Space functions as a bonus round in Scott Pilgrim Vs. The World: The Game, allowing players to rack up a lot of coins. It also serves as the setting for some of the end-game boss fights.