The first section of a C3D file is the header section, which always starts at block 1 - the first block in the file. The first word of the C3D file locates the start of the parameter section in the C3D file, which in turn contains a pointer to the start of the 3D data section as well as a considerable amount of detailed information necessary to read the 3D data section and interpret the contents.
In addition to containing a pointer to the start of the parameter section, the C3D file header also contains duplicates (with certain exceptions) of a number of other parameters stored within the parameter section as well as a small area reserved for storing a limited amount of event information.
The location of the 3D data section, as well as other important parameters, should always be read from the parameter section of the C3D file. The reason that the 3D data section location and other parameters are duplicated in the header of the C3D file is to allow simple utilities to access the 3D data without having to read and decode the entire parameter section.