Many high end audio-interfaces such as the Apogee Symphony systems, are modular. This means you initially purchase a rack-mount chassis, that houses I/O modules or expansion cards. Now you have the option to pick and choose your I/O configuration, by purchasing a specific expansion card that matches your requirements.
Sometimes expansion cards will expand the interface’s capabilities, rather than adding inputs and outputs. For example, the Universal Audio Thunderbolt 2 Option Card, changes a rack-mount Apollo from a Firewire or Thunderbolt 1 audio-interface, into a Thunderbolt 2 audio-interface.