Interior structures are covered structures. Colonists and visitors may pass through these structures without wearing space suits. They may be joined to another inside structure via a connector. Interior structures must have oxygen in them, given by the Oxygen Generator. Every interior structure must have a connection to every other interior structure. Most interior structures require, or are required by, humans to operate, with the exceptions of the oxygen generator, storage, and some Robotics Facilities.
Dead-End structures may only have one connector joined to them, denoted by a 'T' icon. You cannot remove an adjacent structure/connector without first removing the Dead-End structure.