This is used to identify an item/user when importing data. The ID is unique to that item/user.
If you import an item/user into Siteglide without an ID, Siteglide will treat it as a new item/user to be created. It will be assigned a new unique ID automatically.
If you import an item/user into Siteglide with an ID, Siteglide will expect there to already be an item/user with that ID in the database. It will look up the item/user and set the item's/user's properties to the new values you imported. In this way, it's possible to use the export and import tools to update items in bulk.
External ID is simply a text field that must be unique. It is not used to decide which item/user to update.
This field can be used to make it easier to refer to an item from an external system. For example, if importing data from Salesforce then you could set the item's/user's External ID as a reference number from Salesforce (Order ID, User ID, etc.)
If left empty on import, then one will be auto-generated.
Note that this field can only be edited via imports, and not via Siteglide Admin UI