Let me reminiscing little about Refrigeration!

Single Stage = one Compressor. Double Stage (Cascade) = two Compressors. Triple Stage (Compound System) = 3 Compressors No this compressors are not individually double stages.
Single stage Temperature range -50F. With single Refrigerant Recommended; R-404/507/502 or equivalent
Single stage Temperature range -95F. With mix Refrigerants R-12 & CO2. The R-401 can be substituted that is equal to R-12. (134 not suitable) Person must be well acquainted with refrigerants to use substitute in mix systems.
Double Stages (Cascade) temperature range -128F. With refrigerants R-404/507/502 & 503/508B. This temperature can give problem with oil; make sure that your oil is coming back to compressors. Low stage 503/508B most use oil additive. A specially with capillary T. Air/water cool, Capillary or TXV. Above 1HP recommended TXV
Triple Stages Compound system single Refrigerant R-404 or equivalent around -75F.
All system above can be made to operate from around+400F. Down to low number shown and at any giving point, with temperature control of 1Deg. + & -
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