how to define value for primary intended use field in ap invoice worhbench line level.

Hello,

we have done setup in ap invoice that all the tax amount will go to non recoverable tax but in some cases tax is recoverable in that we want to put rule that system only calculate recoverable tax when supplier is VAT registered., for that I am planning to use tax rule - > based on intended use equal to registered. for that I am trying to define primary intended use value.

please guide me to define or navigate step to define value for primary intended use field. in ap invoice worhbench line level.

thans