Being a royal means following protocols and Meghan Markle, much like Kate Middleton did before her, is getting schooled in all the ways of the Royals. From a proper curtsey to the proper royal wave of the hand, there's a lot the Royal family has carried down through the generations when it comes to protocol and tradition.
Queen Elizabeth has one protocol that could be considered a bit on the morbid side, but the reason this was put into place was that a necessity cropped up years ago that caused quite the debacle for a young Princess Elizabeth. The Royals don't leave much to chance but back in 1952 when King George VI died, Princess Elizabeth was not in London, she was in Kenya with Prince Phillip, her husband.
Elizabeth had to fly directly home to take her place as the leader of the country. Now that her father had passed away, she was next in line to take over the throne. When she stepped off that plane in London, she would be seen as the nation's Queen. But due to the King's death, she couldn't step off that plane in anything other than clothes suitable for mourning.
The crowds gathered to see Elizabeth land and step off that plane as their Queen, which she would be as soon as the coronation took place. She couldn't let the masses see her in her vacation attire, so she had to wait on that plane until suitable clothing, a black dress, was delivered by a royal staff member to the plane. She changed into her black mourning clothes and then got off the plane to her new world as Queen.
From this, a lesson was learned and the Queen doesn't travel without a suit of black mourning clothes. According to the Express U.K., other members of the Royal Family don't travel without a mourning outfit of their own with them. This includes Prince William and Kate as well.
With Meghan Markle about ready to enter the Royal Family as Prince Harry's bride, this royal protocol may be something she will need to follow as well. As The Express U.K. said, all members of the Royal Family are expected to follow this one rather morbid protocol.