Client billing and bills for self-pay patients

We are taught that every service must have a diagnosis code to support medical necessity and a hospital must only provide medically necessary services.
Medical necessity isn't consistent and has no conformity.  For any given test, Medicare has diagnosis codes that are considered to provide medical necessity, Anthem, UHC and other insurance companies have their own definition of that medical necessity. 
What about self-pay services?
What about client-billed services, i.e. lab services provided to inmates at a prison and billed directly to the prison?

Are orders required that include diagnosis codes in support of the reason for the test?  I have my own belief based on many years' experience but I'm not able to support my belief with regulations.
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