They very accurately describe information about the telemetry that is downloaded. There is way too much telemetry onboard the spacecraft to be able to down load it all the time. The Flight operations team and subsystem engineers will select which packets are turned on, and how frequently the packets are sent. They will turn on different packets for different activities. Some packets are never turned on unless there is an anomaly.
Mnemonics are pieces of information.
They tell the flight operations team and subsystem engineers what is going on onboard the spacecraft. D. Thruster impulse level is .50 lb. Commands are sent from the Mission Operations Room up to the spacecraft. They are instructions that tell the spacecraft what to do, or set certain parameters onboard. Commands can be sent in real-time where they execute the moment the spacecraft receives them or they can be placed in the spacecraft s memory to be executed at a later time.