The XX in the Backshell part number is equal to the number of signal contacts that can be accommodated. When a Mix-Tek connector is specified with the backshells, each Power or Coax contact is equivalent to four signal contacts. Hence, a 24 contact shell will accommodate six Power or Coax contacts. This table here shows the total range of sizes available for J-Tek signal connectors and Mix-Tek 20 A Power or Coax contacts in straight configurations. The shells are not recommended for use with the Mix-Tek 40 A High Power Contacts.