Default tax code in sap po

Hi All,

I have checked my system in below area

1. MEK1 --Here we mainated few tax codes with combination porg and company codes

2. Company code--we kept V0 tax code

3. Plant we kept --VX TAX CODE

When i create po ...'VA' tax code has come default. System is finding this tax code with combination vendor,porg,company code and plant

Would you please let me know from where this has come.

Regards

Ramakrishnaiah Y