the Atmosphere is what makes the Spacebuild Gamemode have both space and planets, an Athmosphere can contain the four gaseous resources: Oxygen, Carbon Dioxide, Hydrogen, and Nitrogen.
As long as the Atmosphere contains more then 5% Oxygen, and the local temperature is between 283K and 308K players will be able to survive in the Atmosphere without any support devices.
by using a Atmospheric Probe you can find out how much there is of each resource in the Atmosphere.