Most useful in space depends a lot on what you're doing at the time. Docking with small separation? Velocity relative to target. Orbiting something? speed relat

Most useful in space depends a lot on what you're doing at the time. Docking with small separation? Velocity relative to target. Orbiting something? speed relative to that body. Traveling between stars? Tougher but most likely relative velocity towards your target since your path is going to be relatively linear?