In Purchase Order Entry why won't P4310 or P4311 accept the manual entry of 'ST'?

After looking up information about Sales Tax in Procurement I set my Ship-To locations with the Tax Expl Code of 'ST' and the Tax Rate Area and entered a PO. The process worked just fine in my test environment. The Tax Expl Code and Tax Rate/Area where loaded in both the F4301 and F4311 records. I attempted to correct a couple of test Purchase Orders I entered prior to the Sales Tax setup and the programs gave me a 2435 error, Invalid Tax Explanation Code. The error code indicates that any code with the second character of a 'T' is considered "tax only" and can only be entered in Voucher Entry. The PO will eventually make it to Voucher Enter but it will have to detour through Received Not Vouchered.