Positions, velocities and accelerations must be given relative to some coordinate system, which we call a reference system. A reference system where Newton's second law is exactly valid is an inertial system.

A reference system fixed in an accelerating train is not an inertial system. A net force is acting on a body in the train, but the acceleration of the body with respect to the train is zero. So Newton's second law is not valid in this reference system.

A reference system S fixed in the Earth is not an inertial system. The Moon pulls on the Earth, giving the Earth an acceleration which is not present in S.

In a manybody system, a reference system where the center of mass is at rest or moves with a constant velocity is an inertial system.

The velocities and accelerations shown in Orbit Xplorer are given with respect to a reference system where the center of mass moves with the constant velocity (usually zero) given in Parameters.