Answer:

A 30-day Rental Key is a code that may be used to activate Foxhound Version 4 as a Rental Edition copy after

• installing a fresh copy of Version 4 or

• upgrading an existing copy of Foxhound 1, 2 or 3.

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.

The "30-day Rental" price applies to a new, renewed or extended rental period, regardless of whether it is a fresh installation of Foxhound 3, an upgrade from a previous version of the Rental Edition that is being renewed or extended for another 30 days, or an existing installation of the Foxhound 3 Rental Edition that is being renewed or extended for another 30 days. A previous version of the Rental Edition can be upgraded without paying the "30-day Rental" price right away, if there is time remaining in the current rental period.

You can purchase a 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?