US Immigration stamps

Hi All

Does anybody know what the letters 'WT' and 'WB' mean in a US immigration stamp? Are there any other letters that are used and what do they mean?

Thanks

WB (Visa Waiver Business) status or WT (Visa Waiver Tourist) .

The WB is a Waiver for Business classification only for visitors from countries which participate in the Visa Waiver Program.