Question about understanding RTG (Reference Test Gain) in the ANSI S3.22 1996 test

I am reading about the ANSI S3.22 1996 from the document here:

On page 3, last row of the table it says this:

Reference test gain (RTG): Gain when gain control is set to amplify a 60 dB input SPL to (HFA-OSPL90 minus 17dB OR full-on gain OR HFA OSPL90 minus 77 dB.

I have few questions regarding this:

  1. HFA-OSPL90 minus 17dB - Is it saying I should get OSPL 90 curve, then HFA from this curve (average of dBSPL at 1k, 1.6k, 2.5k Hz), subtract 17 from it to getr a certain dBSPL value, then set the hearing aid volume control so that I get the same dBSPL output for an input of 60 dBSPL? If so, then what should be the hearing aid volume level for getting the HFA-OSPL90 in the first place? does OSPL itself imply volume level is always set to maximum?
  2. If HFA-OSPL90 minus 17dB is taken at maximum hearing aid volume control, what does the second option (" OR full-on gain") mean?
  3. Is this same as 1 above, just subtracting 77 instead of 17?
  4. What is the difference between the 3 above? How would I know which one to use?