I don't think i understand what you are asking for but i'll take a shot in the interest of clarifying the question.   

PO items are taxable.  In countries with VAT you pay taxes on the PO lines and offset it against the tax you receive on the Sales (simplistic explanation).  Sage has these taxes on PO line items - green rectangle.

>>select few items are taxable

Assuming this is constant, you can set up taxes for that item.  

An explanation on the first tax field.

This field is used to enter a first tax code to apply to the ordered product.
The tax codes (VAT, additional taxes, special taxes and/or provincial taxes) can be modified on the order line if the transaction authorizes it. They are initialized according to the tax determination rules that most often are coming from crossing the supplier tax rule with the tax levels of the entered product.
If activity code PTX - Regional taxes is activated, it is also possible to access at line level four additional taxes that are used to manage regional taxes. When converting an order line into a receipt or invoice line, the tax codes entered in the order line are transferred to these documents.
The right click is used to select a tax code for each tax level.