What rest frame to choose for when computing the momentum in context of energy–momentum relation

Question

In energy–momentum relation =. From which rest frame is supposed to be captured?

Answer 1

Equation $p=m\gamma \nu$ holds in any inertial frame, where $\nu$ is the velocity of some object of interest in that frame.

Answer 2

The equation can be used in any inertial frame of reference. It must be inertial, those special frames in which these laws are valid.

Reading your comments to some other answers, let me remind you that things like Energy, Momentum, Work, etc. are Frame dependent. They change according to the frame used. But laws like conservation of energy, momentum, etc. are valid in any of these frames. Like in some frame you get your energy to be 20 kJ, then until something do work on you your energy will remain 20 kJ. In some other frame of reference, if you get your energy to be 40 kJ, then your energy will remain 40 kJ in that frame.

Energy, momentum, etc. are just numbers. But the laws are consistent in every inertial frame.