Rotation Relative to

The Rotation Relative to property of an objects sets the reference plane for the object's Obliquity and Argument of Obliquity. There two options for Rotation Relative to: the Simulation Grid, for which the reference plane is the x-y plane of the grid, or the object's Orbital Plane, which is determined by the object's orbital Inclination, Longitude of the Ascending Node, and Argument of Pericenter.

Location
The Rotation Relative to property is located at the top of the Rotation section of the Motion tab in an object's properties panel. If the object has no Orbital Parent, the Rotation Relative to property is not displayed, and the Obliquity and Argument of Obliquity will be calculated relative to the Simulation Grid.