Technically, tiaras are reserved for members of the royal family who are married (that's why you never saw Kate Middleton or Meghan Markle wearing them until their respective wedding days). Tiaras have long been considered a status symbol, but they were also used to discourage men from making advances; if a woman was wearing a tiara, it showed that she was taken and not looking for a husband, and that gentlemen should kindly avoid hitting on her (sigh).
Kate has been photographed in many royal family jewels since becoming the Duchess of Cambridge, and now that Meghan is an official member, she'll likely be able to borrow pieces from the queen's vault for formal events like state dinners and diplomatic receptions.