✓ Accepted answer
SelectionBoxes have a property called Adornee. This property allows you to set what the box is adorned to. By parenting the SelectionBox to the player's PlayerGui, and setting its adornee to whatever you're trying to adorn it to, you can make it so only that player sees it.
Here's a snippet for you (modified from the wiki):
local selection = Instance.new("SelectionBox")
selection.Color3 = Color3.new(0.6,0.6,0.6)
selection.Parent = player.PlayerGui
selection.Adornee = workspace.Part
And that's all it takes. However, local parts[^1] don't work like that; if you want to do this with more than just SelectionBoxes, I recommend you read up on it.
Hope this helped.
[^1]: I'm not a big fan of this wiki page; in my opinion, if you want local parts you should just use FilteringEnabled and create the part on the client, instead of relying on hacky & unsupported techniques.
1 comment
Enable FE and create the object on the client(In a local script).