If you place (travel) expenses on your invoice, you also charge this with the highest VAT rate on the invoice. So, if you invoice for an assignment with two services that are charged with 9% (formerly 6%) and 21%, you will charge the (travel) expenses by 21%.
If you put travel costs on the invoice where VAT is on, such as a train ride, you first deduct the 9% tax from these costs before you put them on the invoice. You may not deduct the VAT as input tax from expenses such as food and drink. You will incur these costs including VAT.