Depends some considerations are type of material being machined, tool life, bio resistance even the type of additives that are acceptable for the end product example Chloro or Sulfur additives may cause part contamination if used on copper alloys.
This is where input from your fluid supplier will be helpful since they know their fluids properties best.
For instance, a “simple” use like a band saw maybe fine with a soluble oil.
Where a high flow high pressure CNC mill with shower wash, through spindle etc. will require a non-foaming long life high rust inhibited fluid.