If you don't want to a redemption value ie. an amount, to appear on your voucher you can only redeem it once ie. one minute it's active, the next minute it's used - there's no ability to allow partial redemptions.

If you want to allow people to partially redeem their vouchers - ie. use parts of the voucher on multiple occasions - you need to add a redemption value. You don't have to offer part redemptions, you can also redeem the voucher all in one go as well.