Tax commitment is only for the PST portion is that correct

During PO entry the amount entered is without any taxes, we enter the Tax Expln code (V/C/B) and Tax area (GST/GST+PST(BC)) etc. the taxes are then calculated when you log a voucher or during Voucher match is that correct and since PST portion is the expense portion only PST portion is the tax committed in F4311. please let me know how and if the above stmt is correct. the Taxes tolerance rules kicks in so the AP admin cannot revise the tax amount over the tax tolerance limit. so the buyer responsibilty is to enter the correct tax area code