The first stage of compression is external; after the refrigerant goes through that stage, it gets pumped to the back of the compressor and then to the second stage of compression. From there, that compressed refrigerant goes to the discharge line and out to the header.

Compound compression systems have a lot of piping. You may notice that suction lines come up and tee off at two compressor heads. There may also be piping that sends some of the refrigerant to other lines. One of those lines is a suction line from the subcooler, and the other is a liquid line that supplies the liquid injection for demand cooling. Both of those lines feed into the intermediary discharge line and use the expansion device element at the discharge line as a reference. (Not to mention, there are several solenoids along the way.)

Racks rely on all of that piping to decrease the compression ratio and improve the efficiency of the rack.

Racks with subcoolers will have liquid coming into and out of the subcooler; suction gas comes out of the evaporator side of the subcooler. The liquid goes to the compressor on demand to keep it cool.