The AFL's Executive General Manager of Football, Laura Kane, said in instances where two or more players look to stand on the mark, the field umpire will nominate which player should stand, allowing others to move out of the protective area.
In the instance on Saturday, Kane said the umpire called for McKay to stand, but the Blues forward seemingly did not hear the call and illegally moved off the mark.
"There's a little bit of common sense that gets applied," Kane told Footy Feed Extra when asked how umpires determine which player should stand the mark.
"If a player self selects and they're the closest to where the mark has been paid and then in fact (is) standing on that mark, it's very clearly them. If two players, which sometimes happens, both go to the spot and the umpire is trying to differentiate who is staying there and who is about to run off, the umpire may select one of the players and instruct them to stay and instruct the other player to get out of the way.
"In the case on the weekend, the umpire has instructed Harry to stay on the mark, but the opposite has occurred and a free kick has been paid.
"It's practical application. If there's one player there, it's very obvious. But sometimes there's not, there's multiple. The controlling umpire closest to the contest and making a decision will make a call and very loudly and clearly tell the players to stay put.
"But in this case, unfortunately, the player has headed off and the 50 metres has been paid."
https://www.afl.com.au/news/1119481/why-contentious-50m-penalty-was-paid-against-carlton-blues-in-geelong-cats-loss"There's a little bit of common sense that gets applied" 
