NRHOs comprise a subset of the halo families of orbits in the Earth-Moon system, characterized by close passages of the Moon and nearly stable behavior.
The NRHO is a highly inclined orbit around the moon and is considered to be in cislunar space. Cislunar is Latin for “on this side of the moon” and generally refers to the volume between Earth and the moon. Cislunar space includes LEO, Medium Earth Orbit, GEO, as well as other orbits, such as Low Lunar Orbit and NRHO, the intended orbit for the Gateway.
Near Rectilinear Halo Orbit (NRHO) example, showing the South L2 example (simplified & not to scale) Credit WestEastSpace.com
Near Rectilinear Halo Orbit visualization Credit MAXAR
An L2 southern NRHO in a 9:2 synodic resonance with the Moon’s orbit around the Earth that completes an orbital period about every 6.5 days. As a destination for the Gateway, the NRHO offers distinct advantages. These include relatively low transfer costs from Earth that fit within the capabilities of the Orion spacecraft, low orbit maintenance costs, and favorable communications opportunities to both Earth and the lunar south pole
Here is a technical presentation done by NASA on the subject.
