If you agree to donate your B unit in the event that your A unit fails, you will be given a voucher.

You can choose one of the following:

either keep the voucher to yourself;
or pass it to your Junior.

At the beginning of each round, you will be informed if your Senior passed a voucher to 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 a voucher can only be used once, and can only be passed once.

But if your Senior passes a voucher to you, 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).

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.