A voucher is a bond of the redeemable transaction type which is worth a certain monetary value and which may be spent only for specific reasons or on specific goods. Examples include (but are not limited to) housing, travel, and food vouchers. The term voucher is also a synonym for receipt and is often used to refer to receipts used as evidence of, for example, the declaration that a service has been performed or that an expenditure has been made.
— In-Game Description
Vouchers are a gameplay feature in Battlefield Hardline, available in Battlepacks. Once obtained, they allow the player to unlock the specified item for any weapon, agent or vehicle, though this may only be used once per voucher. For instance, the PK-AS voucher would allow the player to unlock the PK-AS holographic sight for one weapon of their choosing for no extra cost. Once used, however, the player may not use it again to unlock another PK-AS for a different weapon. They can be sold for a small amount of cash.
Vouchers may be used before unlocking that sort of attachment. e.g. A stubby grip voucher may be used before getting 90 kills with a specific weapon. Weapon License vouchers are also available in Battlepacks, allowing faction specific weapons to be used by both factions.