A satellite is an additional portion of the machine that runs at a slightly higher or lower suction pressure.
Typically, the idea is to save energy by only using the capacity that is required to maintain the colder temps or by grouping the higher temperature circuits together and running them at a higher pressure.
I'm sure you understand compression ratio: The ratio of absolute discharge to absolute suction (with absolute being gauge pressure + 14.7, if at sea level).
The higher the compression ratio, the less efficient the compressor is. This has to do with the amount of piston travel needed to re-expand the vapor that is trapped in the clearance between the top of the piston and the bottom of the valve plate.
So, if you can isolate the system operating at the lowest saturated suction temperature by piping it to its own compressor (the satellite compressor), the remainder of the compressors on the rack can operate at a higher saturated suction temperature, meaning more efficiently. No need to operate the entire rack at -25 SST to satisfy the 24' of ice cream cases. Use a satellite for this, and operate the rest of the rack at whatever the FF cases require...say -15 SST. This translates into less $$ on the power bill at the end of the month.
Here's an example:
A 3D compressor operating at -25 SST and 105 SCT has a capacity of 46,300 Btu. The same compressor operating at a -20 SST and 105 SCT has a condenser 50,500 Btu. That's a 10 increase in capacity, simply by operating at a higher SST.