Heat
Heat is a waste byproduct of nearly everything aboard a space station. As such, it needs to be managed carefully to avoid overheating the station and its occupants.
Heat "supply" actually represents the station's ability to remove excess heat, while "demand" represents the Heat that is actually generated in the station; in this sense it is "backwards" from other resources, though aside from this logical discongruity it is still treated identically. Excess Heat will make aliens unhappy, and can lead to systems failing; aquatic aliens are most adversely affected by excess Heat.
Heat is generally managed by radiators, which unlike other systems must be built on the outside of the station; they can, however, extend beyond the extents of the game world. Radiators will have 1 or more "root" tiles that must be within the game's grid and adjacent to station tiles, and multiple tiles visually representing the radiator itself which may extend into areas otherwise inaccessible for building.
Some radiators may be expandable, in which case adding additional Heat capacity is a simple matter of interacting with a UI attached to the "root" tile(s), which will further extend the visuals.