Biometric Factor.

The biometric factor forces the recipient to prove their biometric test on their own device to access the share. Much like the personal factor, this is attached to the share itself and requires bio access to be accessed on a device, now or later, during the lifespan of the share.

Authors simply tap biometric, and slide to enable, and save. 

If the recipient has biometric disabled, they will not be able to access the share.

If the recipient falls back to the OS PIN in lieu of the biometric scan/touch, the OS will allow this as a substitute. This flow is out of Foursure's control.