Unlike the orderly or batman, a humble low-ranking servant, an aide-de-camp is often a general, who before the institution of the chief of staff could hold a similar position to his chief.
Then ‘the ADC in Waiting was commanded by Their Excellencies, Lord and Lady Lytton, to invite Mr and Mrs Cusack-Bremmil to Peterhoff on July 26 at 9.30 p.p.’.