Purchase Tax Rate is specific to expenses and so defined in the Expense Items detail view at the individual expense item level. If most of your expenses have a single purchase tax rate, say 15% HST applied, and only a few have a different rate (say airline tickets), you can make use of the Batch Change feature under the Expense Items > Actions menu to update the Purchase Tax rate of all expense items to that single value. Then you need to edit the rate of only those few items that have a different rate.
You can enter purchase tax as an amount or a percentage. For amount, you must enter the dollar symbol ($) followed by the purchase tax amount in the Purchase Tax Rate field. CORE reverse calculates and automatically converts it to a percentage. Check CORE Help Center for details.