A reference body is a body in relation to which a change in the position of other bodies in space is considered.
The reference body is associated with the coordinate system, which is a reference point. To determine the position of a body in space at any time, you must set the start of time.
When solving each specific problem, choose a convenient frame of reference and a convenient coordinate system, and the clock (ideal) in each frame of reference need only one. However, the reference body, the origin, and the direction of the coordinate axes are chosen at random.