RBX.lua.StarterGui (Service)
From Legacy Roblox Wiki

The StarterGui object is a service-level object that holds GUIs. GUIs inside this object get duplicated into a Player's PlayerGui when they spawn.
Functions
The StarterGui object only incorporates global functions.
Properties
In addition to the global properties, the StarterGui object also has these:
Events
The StarterGui object only incorporates global events.