Telemetry is a transport medium. The satellite collects data into a buffer, and then divides the buffer into smaller pieces for transport. The ground systems must then re-assemble the packets into a representation of the buffer so that it can be interpreted.
As an example, the hardware for the X-Ray Timing Explorer was designed to transmit quite a bit of status information for verification, with the idea that some of it would be useful downstream. It is still being decided how the ground operation team will use all the information.
The need to register multiple handlers can be made unnecessary by defining a handler object that performs some arbitrary list of functions on the complete unit of work. The UNIX atexit() function is used to register a handler to be called as the process exits. By calling atexit() with a pointer to a function a user adds an additional handler to be called when the process exits.