Personal Aide-de-Camp

A Personal Aide-de-Camp is a senior military officer who is appointed to act as the honorary military attendant to the monarch of the United Kingdom and the other Commonwealth realms or any of his or her viceroys. The Sovereign will typically commission another member of the Royal Family to act as his or her personal aide-de-camp, though other non-royal officers will be assigned to the role, especially when the monarch is in one of the realms besides the United Kingdom. A personal aide-de-camp to the sovereign is entitled to the use of the post-nominal letters ADC(P), while those designated as aide-de-camp to a governor general, lieutenant governor, or governor use the letters ADC or A de C after their names. The emblem of the office is the aiguillette.

Those in the Royal Family who currently hold the appointment are:

There are other categories of aides-de-camp to the Queen; most are serving military, naval, and air officers, usually of colonel or brigadier rank or equivalent. There are also specific posts for very senior officers, such as First and Principal Naval Aide-de-Camp, Flag Aide-de-Camp, Aide-de-Camp General, and Air Aide-de-Camp each with its own specific entitlement to post-nominal letters.

...
Wikipedia