The FORCE_PLATFORM:USED parameter is a single signed integer value that records the number of force platforms for which analog data and parameters exist in the C3D file.  This may contain any value between 0 and 32,767 although it is possible that some applications will have problems interpreting data from large numbers of force platforms.  Most applications seem to support at least four force plates.

If FORCE_PLATFORM:USED is set to zero, then the remaining force platform parameters are not valid.  It is important that the USED parameter exists even when the C3D file does not contain any force platform information – this allows older software applications reading the C3D file to determine that force platform information does not exist.