Definition of "confirmatory"

I replied, as I usually did whenever I had a chance, that nobody had ever loved anybody else as I loved Dora. Traddles came to my assistance with a confirmatory murmur.

1850, Charles Dickens, chapter 41, in David Copperfield‎

"Do you come to me, Master-at-arms, with so foggy a tale? As to Budd, cite me an act or spoken word of his confirmatory of what you in general charge against him. […] "

1924, Herman Melville, chapter 16, in Billy Budd‎, London: Constable & Co.

The main indication for performing the DST is for the confirmation of suspected major depression. A positive test is excellent confirmatory evidence of the diagnosis.

1986, Mark S. Gold and A.L.C. Pottash, Diagnostic and Laboratory Testing in Psychiatry, New York and London: Plenum Medical Book Company, Chapter 3, p. 43