Good day

i want to be cleared the concept of Vi of screw compressors. some compressor have fixed and variable VI.
And they have calculated figures like 2.6 3.6
Some typical screw compressor Vi ratings are 2.6, 3.6 and 5.0. A low Vi (2.6) typically is used on booster applications while a high Vi (5.0) is used on high- or single-stage applications.
this calculation on which parameters reference calculated ?

Thank you all