For example, in the past, Princess Beatrice would have had to ask Her Majesty before becoming engaged.
However, when she became engaged to Edoardo Mapelli Mozzi this month, she did not have to seek her grandmother's permission.
When Kate Middleton and Prince William become King and Queen, their children will face different requirements when it comes to marriage.
According to Express, MP Christopher Pincher asked "Where did the number six come from? Why not three, five, or 12?"
"If we put in place a rule which says that the monarch can and must give consent to the marrying of the six persons nearest in line to the throne, imagine a scenario where a monarch has three children, who each have two or three children.
"The monarch will soon be in the invidious position where grandchild number four, who is fifth-in-line to the throne, must seek consent of the monarch to marry, but grandchild number six, who is seventh-in-line to the throne, need not seek that consent.
"That does not seem fair."
Due to the fact that Prince William has three children, he would be put in the "unfair" situation and will have to grant permission to some of Princess Charlotte's children but not all.
What's more, Princess Charlotte would require his permission to marry but Prince Louis wouldn't.
Presently, Prince George is third in the line of succession to the throne, behind his father.
Princess Charlotte is fourth, and Prince Louis is fifth.
Prince Harry is sixth-in-line, however, if William and Kate have more children, the Duke of Sussex will move further down the line.
According to Express, the number six became law in the 2013 Act because, in recent history, the throne has never passed to anyone who was more than six steps away from the line of succession.