Relativistic mass is, as the name suggests, relative. It depends on your frame of reference. If you observe a spaceship traveling at .9C, for example, then from your perspective its mass will be significantly greater than from the perspective of the passengers. This doesn’t affect its intrinsic mass, if that’s what you mean by “actual” mass, but it’s still a measurable phenomenon. Intrinsic mass and relativistic mass are interchangeable, such as when a nuclear reaction releases products with a lower rest mass than the reactants, at high atomic velocity.
Relativistic mass is, as the name suggests, relative. It depends on your frame of reference. If you observe a spaceship traveling at .9C, for example, then from your perspective its mass will be significantly greater than from the perspective of the passengers. This doesn’t affect its intrinsic mass, if that’s what you mean by “actual” mass, but it’s still a measurable phenomenon. Intrinsic mass and relativistic mass are interchangeable, such as when a nuclear reaction releases products with a lower rest mass than the reactants, at high atomic velocity.