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.