I was learning about this when I noticed that StarterGui/PlayerGui is not listed among the instances where server Scripts will run.
https://developer.roblox.com/en-us/articles/Roblox-Client-Server-Model
Yet on this page: https://developer.roblox.com/en-us/api-reference/class/PlayerScripts
It is mentioned in passing that the server has access to PlayerGui
I did a test, putting a script changing the time of day in StarterGui, which ran and changed the time of day on the client and the server.
The script: game.Lighting:SetMinutesAfterMidnight(0)
So shouldn’t the Roblox Client-Server Model-page be updated to also include StarterGui/PlayerGui?