The Deputy Lieutenant is the deputy to the Lord Lieutenant of a county. In titles, the suffix DL may be added; e.g. John Brown, PC, DL. However, it is generally omitted if the subject has many honours and titles. Ensign of the Lord-Lieutenant The title Lord-Lieutenant is given to the British monarchs personal representatives around the United Kingdom. ... Originally, a county was the land under the jurisdiction of a count (in Great Britain, an earl, though the original earldoms covered larger areas) by reason of that office. ...
A "captain lieutenant" in the British army was formerly the senior subaltern who virtually commanded the colonel's company or troop, and ranked as junior captain, or "puny captain," as he was called by Cromwell's soldiers.
The lord lieutenant of a county, in England and Wales and in Ireland, is the principal officer of a county.
A deputylieutenant (denoted frequently by the addition of the letters D.L. after a person's name) is a deputy of a lord lieutenant of a county.