This add-on is for a portable, hard plastic, milk-carton-shaped deck box. The box can store approximately 210 sleeved cards (in the Seize the Bean size of 75x50mm) which is roughly a full-game setup of Seize the Bean. If purchased in a pair the other deck box could fit the majority of other components (minus the resources, which would be stored in resource bags nicely, if those were also purchased). As can be seen in the photos (which are of a manufacturer's sample) there is a secret compartment in the lid for extra storage.
