T0 Zero Rated P/L and S/L (No VAT) T1 Standard Rate T5 5% (Lower Rate) (Insurance PT Electricity, Gas Bills) etc T9 Outside the scope (Wages/Salaries) etc...
as importantly the VAT codes determine the box numbers figures appear in on the VAT return produced on Sage...
I also use T2 for exempt items like postage and T0 for items that are taxable but taxed at 0% (pushed to come up with an example at the moment!).
I know this has been the subject of much discussion on here, but I have now taken to using T1 with No VAT for suppliers that are not VAT registered, in the event that they would be taxable supplies if they were registered.