Understandable that a person can need only localized SRP's (4342), however, that was never mentioned and left to interpretation. I did not know that one sterilizes subgingival when doing SRP's whether localized or generalized. I thought that it is done to remove toxins, bacteria, biofilms, calculus (which isn't just a math) and necrotic and/or infected tissue. I guess in a sense it is sterilizing especially if one uses chlorhexidine gluconate as subgingival irrigation in the process.
And I differ in the perception that a prophy is "essentially" polishing of the dention above the gumline. A regular prophy (1110) is supragingival yes but also intales going 1-2 mm subgigival to ensure proper and thorough removal of calculus and plaque with hand instruments (scalers and currettes). The polishing aspects of the "prophy" only ensures a smoothing of the enamel surface.