In the context of special relativity, there is no absolute and objective notion of which frame is "moving" or "stationary" when considering two inertial frames of reference. The principle of relativity, as formulated by Albert Einstein, states that the laws of physics are the same in all inertial reference frames.
In special relativity, motion is always relative, meaning that the description of motion depends on the observer's perspective. Two observers moving relative to each other at a constant velocity cannot distinguish who is "moving" and who is "at rest" based on the laws of physics alone. The concept of "moving" or "stationary" is relative to the observer's frame of reference.
This is encapsulated in the principle of the constancy of the speed of light, which is a fundamental postulate of special relativity. According to this principle, the speed of light in a vacuum is the same for all observers, regardless of their relative motion. As a consequence, the observed behavior of time dilation, length contraction, and the relativity of simultaneity depends on the relative motion between the observer and the observed object.
Therefore, in special relativity, the determination of which frame is "moving" and which is "stationary" is a matter of perspective and depends on the choice of reference frame. The laws of physics remain consistent in all inertial frames, and there is no preferred frame of reference.