If you and your friends play soccer in an open area of the park, probably all agree that other people should avoid setting up their picnic in the area of play. Parents and dog-owners should keep their kids and pets from entering. If you and your friends declare the whole park to be the area of play, including the playground, children's garden, bathroom, splash pad and parking lot as shown in the finished map, I think this ceases to be a reasonable expectation, but what the norms should be is ambiguous.