I made a script that creates a selection box when the mouse hovers over a part, but when I do server testing the second client can also see this. I tried placing the code in a LocalScript and it didn't even work at that point.
Not sure if you need the code or if this counts as a request.
But here is my code:
script.Parent.ClickDetector.MouseHoverEnter:connect(function(Player)
local sb = Instance.new("SelectionBox")
sb.Adornee = script.Parent
sb.Parent = script.Parent
script.Parent.ClickDetector.MouseHoverLeave:connect(function(Player)
sb:Remove()
end)
end)

2 comments
Use filtering enabled or if you don't want to use that then (hacky method not recommended) parent anything you want to be local to the camera (in a localScript obviously).
I am using filtering enabled, but I had it in a server script before that so that's why I was going to start to parent it into the camera.