Esquire, or Esq. as an abbreviation, was used to denote a man who was regarded as a “gentleman” but not otherwise of a specific rank.

Amongst retired officers it was regarded as “bad form” to use their former rank as a title in civilian life unless of Field rank (i.e Army Major or above).

Thus a retired Army Captain would be unlikely to describe himself as Captain Smith in civilian life, whilst a retired Navy Captain (equivalent to a full Colonel ) would call himself Captain Smith R.N.