Field help (GDAVAT) 

Enter the tax code to be applied to the line, when calling the structure in journal entry mode or in BP entry mode.
The field is initialized by default with the tax code of the account, but the value remains modifiable.

If the SAITAX parameter is set to No, the tax code is not mandatory, while it is when the SAITAX parameter is set to Yes.
To be noted, the initialization conditions of the tax code when SAITAX is set to No: