1. Most efficient altitude for ground distance per unit fuel used fitting in with ATC requirements.
2. ATC requirements & fuel efficiency above.
3. Speed of sound = 644 x 1.2T kts (T= deg C). So as C gets colder and more negative, speed of sound goes down.
4. At low alt, air warmer, therefore Speed of Sound is higher, so M.84 will have a higher TAS
5. The whole point is to make operation most economical in relation to fuel consumption/aircraft time cost/commercial requirements/curfews. A pragmatic approach to all the above is required 'on the day'.