Because the definition of r SOI relies on the presence of the Sun and a planet, the term is only applicable in a three-body or greater system and requires the mass of the primary body to be much greater than the mass of the secondary body.
The term is only applicable in a three-body. The term is only applicable in a three-greater system. The term requires the mass of the primary body to be much greater than the mass of the secondary body. The definition of r SOI relies on the presence of the Sun. The definition of r SOI relies on the presence of a planet.