Similar to a lifeboat on a cruise ship, the spacecraft isn't expected to be called into service to quickly evacuate people but it certainly has to be ready to do so just in case.
Currently the lifeboat function of the space station is served by a pair of Russian Soyuz spacecraft, which are docked at all times. Each Soyuz can hold three people, so with two of them docked, there can be six people working on the station at one time.
According to NASA engineers working with companies developing spacecraft in the agency’s Commercial Crew Program (CCP), in order for a spacecraft to be considered a lifeboat, it must provide a shelter for astronauts in case of an issue aboard the station. The ship must also be able to quickly get all of its systems operating and detach from the station for a potential return to Earth.
When it comes to the lifeboat feature, two obstacles that make it difficult for spacecraft designers are power and protection from things outside the spacecraft, such as micrometeoroids. The electricity generated by the space station’s acre of solar arrays is reserved for the station’s systems and science experiments, and the amount of power available for a docked spacecraft isn't much — it's similar to the amount of electricity a refrigerator uses.
Designers also have the challenge of building a spacecraft strong enough to withstand impacts from micrometeoroids, but at the same time they cannot carry armor that’s too heavy to launch.
CCP gave aerospace companies a list of requirements their spacecraft need to meet during NASA’s certification process for use as in-orbit lifeboats. Boeing, Sierra Nevada Corporation and SpaceX are working in partnership with NASA on spacecraft designs that meet these requirements.