Whats products supplies are exempted from the sales tax withheld?

There are different types of withheld taxes and the predominantly used model is VAT in many tax regimes, hence under VAT system there is something called Zero rated (i.e No tax) and the types of products and services that comes under the zero rate varies depending on country to country.

For example in  many countries education  related products comes under zero tax rate

However  to give a precised answer you shall mention the tax regime.