Those categories are accounts in the chart of accounts. You can add accounts to your chart of accounts by clicking on "Accountant & Report Tools" on the top-bar and then Chart of Accounts. Add a new expense account there, and you will be able to select it in the category drop-down when creating bills.
Hi Siobhan, The code is a unique combination of characters that identify the account. From your point of view, the code doesn't really matter as long as its unique - you can make it the same as the account name if you like. John.