Speaking to news.com.au, Meier ran through what was deemed appropriate for the royals.
Pantyhose are a must and open-toe shoes are unacceptable for day time events. Another strict daytime rule is that apart from marital or religious jewellery, diamonds are unaccepted for a royal while working.
"Other jewels are worn pre-6pm," Meier explained.
"Before 6pm, you'll see metallics, gemstones, pearls, sapphires. At night, you'll see diamonds come out,"
The reasoning behind this rule is that it ensures the royals do not come across as "flashy in [their] appearance."
When it comes to tiaras, Meghan has only worn one at her wedding to Prince Harry in May 2018. Meanwhile, Kate has donned a tiara a number of times for state occasions since her wedding to Prince William in April 2011.
For skirts and dresses, the duchesses are expected to have "modest hemlines and slits that are only for function," explained Meier.
Discussing slits for formal daywear, Meier elaborated with, "There might be one in the back or a slight, slight one in the front to help them walk, but slits are for function, not for style during the day."
Meghan has often come under fire for skirting around one rule in particular... bearing her shoulders.
"At formal engagements, we typically always see the shoulders covered," Meier said, "It's expected shoulders will be covered."