config: If you have multiple voucher configurations, which allow you to issue vouchers valid for different sets of top level domains, then specify the name here. If you don't provide one, then we default to your main configuration.
days: the number of days before the voucher expires. We set this to 365 by default, but you can choose any lower value.
callback_url: a URL where you can be notified when a voucher has been successfully redeemed. Hover sends HTTP POST to the callback_url with the output of https://www.hover.com/api/vouchers/:id in the body.
metadata: a value (either a string or JSON pairs that is stored with the Voucher and passed back when getting info about the Voucher and in the payload sent to the callback_url and via webhook. This is an optional parameter, enabled on a per-Partner basis by request.

When you issue a voucher, the API returns a subset of information, but with two extra fields that do not appear anywhere else:

The code field is the secret name of the voucher, which the user will need to redeem it. We do not keep a copy of this (it's one-way encrypted at the time of issuance), so keeping track of it is your responsibility.
The redemption_url is the same voucher code in the form of a full URL that your customer will visit to redeem the voucher.