The POINT:USED parameter is normally an unsigned integer that contains the number of 3D point coordinates that are written to each frame of data in the C3D file data section.  If it is wished to store coordinates for ten 3D points, then POINT:USED must be ten or greater, and every 3D frame will have space for POINT:USED number of 3D points.  This parameter describes the number of points that are stored in every frame of data in the C3D file and is used to enable the 3D data section of the file to be interpreted.  Every point in the C3D file should normally have an associated entry in the POINT:LABELS and POINT:DESCRIPTIONS parameters.

The importance of the POINT:USED parameter lies in the fact that an application reading the 3D data section directly uses this value to determine the number of 3D coordinate points stored in each frame.  The points do not have to be valid, they just have to have storage allocated; invalid points should be stored with a negative residual if no valid trajectory data exists.  When an application has read POINT:USED number of 3D coordinate points then it has read the entire frame of 3D data.

A copy of the USED parameter value can also be found in word 2 of the C3D file header.  The POINT:USED header value must always be identical to the parameter value.

While the use of an unsigned integer to store the number of points in a C3D file means that a maximum of 65535 points can be stored,  the associated POINT:LABELS and POINT:DESCRIPTIONS parameters are limited to a maximum of 255 entries.  This limit can be bypassed by creating additional LABELS2 and DESCRIPTIONS2 parameters.