Why does it not make sense to talk about the existence of only an electric or only a magnetic field, without indicating

Why does it not make sense to talk about the existence of only an electric or only a magnetic field, without indicating with respect to which frame of reference these fields are considered?

At the same time, the charge can be at rest relative to some reference frames and move relative to others. While resting, he will create only an electric field, and while moving, he will also create a magnetic one. Electric and magnetic fields are manifestations of a single electromagnetic field.

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