In Factorio the term "global" refers to 2 different things:
localare global: https://www.lua.org/pil/14.2.html.
globaltable each mod has access to - a table with the name
globaldeclared as a global variable that is persisted between map sessions.
During the lifetime of a mod it's frequently required to save mutable data. Although mods are free to store such data in any variable the game will not persist that data between saving and loading.
To address this Factorio provides the table
global - a global table - that is serialized and persisted between saving and loading of a map. Each mod has access to its own instance of this table (
global) so data stored does not need to use some unique name.
Only specific data can be saved and loaded using this table:
Circular references are also handled properly.