Answer:

A 30-day Rental Key is a code that may be used to activate Foxhound Version 3 as a Rental Edition copy after

• installing a fresh copy of Version 3 or

• upgrading an existing copy of Foxhound 1 or 2.

A 30-day Rental Key may also be used to extend or renew the rental period for an existing installation of the Rental Edition for an additional or subsequent 30-day period. A different Rental Key is required for each 30-day period; see the Renew or Upgrade Foxhound button on the About page.

You can purchase an 30-day Rental Key here:

See also...
What are the differences among the different editions of Foxhound: Rental, Basic and Extended?
What are the differences among all the activation keys?