Its function is simple: A structure or item within range of the campmarker cannot be lit on fire with a Torch or Lighter, and cannot be hammered unless it's the person that placed down said campmarker. Everything is still flammable, but it would be harder for a player to torch something if it's protected by the campmarker.
If the player who places down the campmarker leaves the server, the campmarker can be dug up by anyone. After that, the structures are vulnerable to hammering and torching.
When investigated, it would show the character that placed it down, similar to investigating skeletons. It would also say "[player]'s Campmarker" when hovered over. Quotes would be something like this:
Wilson: "This is [player]'s camp."
Willow: "I shouldn't light [player]'s camp on fire."
Wolfgang: "Wolfgang likes [player]'s home!"
Wendy: "Maybe I won't be alone at [player]'s camp..."
WX-78: "[player] ISN'T USELESS AFTER ALL"
Wickerbottom: "Ah, marvelous campsite, [player]!"
Woodie: "[player] makes a fine camp, eh?"
Maxwell: "[player] knows what they're doing."
Wigfrid: "Shall I defend [player]'s campsite?"
Webber: "[player] knows how to make friends!"
more to be added later