Dear Carla,
You can find the definition of vaccine storage capacity in the PQS specifications:
http://www.who.int/immunization_standards/vaccine_quality/pqs_e04_insulated_containers/en/index.html
Vaccine carrier:
Vaccine storage capacity: The total volume of the vaccine storage
compartment, in litres. The measurement is equal to the volume of the largest
rectilinear object that can be inserted into the compartment with all the
manufacturer's specified packs in place.
Cold boxes:
Vaccine storage capacity: The vaccine storage compartment is loaded with
cardboard boxes or blocks made out of plastic foam or wood measuring
100x100x100 mm or 100x100x50 mm, packed so that there is minimal air
space between each column of packets or between the units and the ice-pack
lining. The total volume of the test units, in litres, represents the net volume
available for the storage of vaccines.
Denis Maire
WHO PQS Secretariat