When it comes to a public display of affection or PDA, there are some unwritten rules that people tend to follow, but for the Royals, there are even more do's and don't's to keep in mind. This is something Meghan Markle has had to learn first hand since the moment she and Prince Harry started dating, and for a non-royal, it is safe to assume it is hard to memorize all the rules. With different types of events and engagements, there are also different sets of protocol to be followed, and this also involves when to show PDA.
Probably because the public never used to see much royal PDA in the past, everyone tends to love seeing snapshots of the royals being cute and affectionate with each other. No one can forget seeing Prince Harry telling Meghan she looked beautiful at the altar of their gorgeous and star-studded May wedding earlier this year, and whenever the newlyweds and parents to be lean in to steal a kiss, media goes into a frenzy.
As reported by Cosmopolitan, the Royals generally adjust the amount and type of PDA to how formal the event they are attending is. If it is a completely informal and perhaps non-work related event, they are free to show as much PDA as they want - within the frames of what is socially acceptable of course. However, if they are attending a very formal event where they are representing the Royal family, they are not likely to show much PDA at all.
These rules were much stricter in the past, and still to this day the Queen and Prince Philip are never seen sharing much physical contact at all, apart from the very occasional hand-holding. With the younger royals, the rules are a bit more relaxed, but because of the Queen's standpoint, Prince William and Prince Harry choose not to hold hands with their wives in front of her out of respect. There are some moments in which the Royals are encouraged to show affection to their spouses by kissing on the lips. An example of this would be at the charity polo matches, and of course at their own wedding ceremonies.
The royal PDA rules are fairly similar to those of anyone; at a formal event or a work-related engagement, keep PDA to a minimum, whereas at a private event or a friend's party couples in love are free to get a bit more hands-on lovey-dovey.