A voucher can last for at most two rounds. If it is not used in the first round (i.e., not used by the donor who generates it), it can be passed to the donor's Junior and the Junior can use it in the second round.

So there are two ways to get a voucher:

If your Senior is a donor (so has a voucher) and did not use the voucher in the previous round, you will inherit the voucher and can use it in the current round. At the beginning of each round, you will be informed if you inherit a voucher.
If you agree to donate your B unit in the event that your A unit fails, you will be given a voucher.

Note that a voucher can only be used once, and can only be passed once.

But if you inherit a voucher from your Senior, and if you become a donor, you are holding a voucher yourself (which you inherit from your Senior), and your Junior will also inherit a voucher from you (which is generated by you).

The voucher will give you a higher priority for the allocation of donated B units in the event that your B unit fails and you are waiting for donation from other players.

In other words, the sequence of B unit allocation is as follows:

If any donors had A unit failure, their B units are allocated to the players who have B unit failure and have vouchers.
If there are any donated B units remaining, they are allocated to the players who have B unit failure but do not have vouchers.

Note that for each round, a player can use at most one voucher. In other words, holding two vouchers does not give you a higher priority than those who have only one voucher.