Hi Swetha,
Which tax numbers goes where is mostly dependent on how you set up the system. The tax numbers in each field that is suggested by SAP can be discovered by doing an F1 on the fields. Eg, TAX No1 is suggested to be the SSN for US, TAX NO2 is the Employer Identification number.
The VAT fields are mainly for EU countries. Here there is some transactional relevance, if a VAT number exists for the shipping and delivering country, the delivery will be tax exempt. This check rule is hardcoded and cannot be changed.
Please take a look at this discussion also:
What is te relevance of VAT registration number in Vendor master Reord?