SB - I think the question is a little confused? I'll try to answer what I think you ask.
If you are flying above the level of the jetstream, it MAY be advantageous to descend to take advantage of a higher tailwind.
It depends on relative wind speeds and the change in cruise level. Performance charts for your 'one fast jet' (which from your profile you will have access to?) will tell you. If you are flying BELOW the jet, generally a climb will be beneficial for an improved tailwind.
All this depends on:
Turbulence/weather at jetstream level, and
How far above/below 'optimum' cruise level you will cruise.
QNH etc do NOT have abbreviations - they ARE in a sense 'abbreviations'. They came into being during the days of Morse code to avoid sending too many letters.