What is the voucher for wireless clients?

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A voucher is a unique printable code that allows a user to authenticate with a Wi-Fi network for a limited period of time. A user connects to the Wi-Fi network’s SSID and then enters the code in a captive portal. After a successful login, the expiry time of the voucher starts counting down.
Vouchers are useful in situations where you want to give individual users time-limited Wi-Fi access. For example: A customer can purchase a voucher for 2 hours of Internet access in a hotel or coffee shop.

To know how to configure captive portal with voucher, please refer to this FAQ