Global picklist value sets
Key Takeaway
A global value set is a shared, always restricted list of values, so one change to the set applies to every custom picklist field that uses it.
A global picklist value set (a global value set for short) is a list of picklist values that exists on its own, outside any single object. A custom picklist field, by contrast, is tied to one object. Many custom picklist fields, even on different objects, can share one global value set. App builders use it to keep values consistent across objects and to limit picklists to values an admin has approved.
Global value sets are always restricted
Global value sets are always restricted picklists, and you can't convert them to unrestricted. Only a Salesforce admin can add or change their values. Users can't add unapproved values, even through the API, which protects data integrity. Creating or changing a global value set requires the "Customize Application" permission.
That restriction is the trade-off for using a global value set. A global value set gives efficiency and consistency across many picklists. Independent custom picklist values are more flexible, but they take more upkeep.