Anonymous vouchers are vouchers, for which any user or device identifier sent to TROPHiT as part of the underlying integration, are never used or stored by TROPHiT in any way. Currently, you only use Anonymous Vouchers for Facebook-driven ad campaigns, as Facebook does not allow sharing of any user-specific identifiers by advertisers with 3rd parties (part of Facebook's Data Use Terms for Advertisers).
To set a voucher as Anonymous, check the "Anonymous" checkbox in the voucher setting page in your TROPHiT dashboard.
Since TROPHiT uses user or device identifiers to enable its Redemption Control features, anonymous vouchers do not support Redemption Control and are therefore recommended only for redemptions by new users or content that, by nature, is only a one-time thing, like "unlocking of the Insane difficulty level".
Note: despite the fact that anonymous vouchers don't require any user identity to be provided in the attribution data in order to perform redemption, such identity is not completely ignored in all cases: if the voucher is associated with a segment that requires reservations, it will only be redeemable if the user can be identified from the attribution data (however, like with any anonymous voucher, no record of that identity is associated to the redemption)