User Defined Categories can be called whatever you want so it may be labelled different even between different companies within the same database.
To update your Customers or Suppliers User Defined Categories you'll need to use the Update Customers/Suppliers tool. This can be found usually in one of two places either:
From the toolbar, System Admin, Setup, Update Customers/Suppliers
From the toolbar, System Admin, Data Imports, Update Customers/Suppliers
Despite the option sometimes being in the Data Imports screen, no data needs to be imported. To use the screen:
Select the Ledger type, either Sales for Customers or Purchase for Suppliers.
Populate the left side with data you wish to filter on.
On the right, enter the Customers or Suppliers data to update. Here you can specify the different User Defined Categories to your needs.