Field help (GACVAT) 

By default, each account can have up to three tax accounts that determine the tax calculation, posting, and declaration terms. For more information, see the documentation on Tax rates.

The first code to enter is the VAT code. The two other codes must be a different type.

If the SAITAX - Mandatory Tax Code(CPT chapter, VAT group) parameter value is set to Yes, it is mandatory to specify at least one tax code in account creation mode. If the SAITAX parameter is set to No, the account can be created without a tax code. The tax code is then searched for via the invoice tax rule in miscellaneous table no. 1, "Tax code" field.