In the jurisdictions where tax/VAT is levied on billed amounts,  during invoice generation (either Bulk Invoice Generation or Individual Invoice Generation), VAT/Tax rate may be entered.

However, in some jurisdictions, out of pocket expenses may not be taxable.  In such cases,  during individual invoice generation (using Create Invoice button under Cases (Projects)-> Invoices tab for a selected case), after adding notes (time/expense entries to the invoice popup, uncheck "Taxable" checkbox from expense entries and then enter the tax/VAT rate before clicking on Generate Invoice button.

Generating invoices using Bulk Invoice Generation (under Settings/Tools/Reports),  enter Tax/VAT rate and then uncheck "No Tax/VAT on Expense Entries" box.