The POINT:LABELS parameter is a character data array that consists of one unique four-character ASCII value for each 3D data point contained within the C3D file.  By convention, the parameters are usually four characters of upper-case standard ASCII text (A-Z, 0-9).  The contents of each LABEL (e.g.  LASI, RASI, LTOE etc.) is referred to as the point label and is used to reference each 3D point contained within the C3D file data section.

In the original C3D file description (signed C3D), arrays use a signed byte as an index.  Signed bytes have a possible range of –128 to +127 but since the array index is always a positive number, the maximum number of array entries is 127.  Since negative array indexes are illegal, the range of the array storage can be extended by interpreting the index as an unsigned byte with a range of 0 to +255.  The use of an unsigned byte for the array index can be assumed if the array index appeared to be negative when read as a signed byte.

The purpose of the LABELS parameter is to allow applications to search for a specific 3D point or trajectory by referencing its LABEL value instead of looking for a specific trajectory number in a fixed list of trajectories.  This allows applications to be written in a general manner so that they can process data by reference e.g., calculate the direction of progression from the 3D points identified as points LASI, RASI and SACR.  An application written in this way will work in any environment, as it does not require that the 3D data is stored in any specific order within the C3D file.

The POINT:LABELS parameter can refer to a maximum of 127 3D data points in an “unsigned C3D file” or a maximum of 255 3D data points in a “signed” C3D data file.  Note that a C3D file may contain more or less than the number of trajectories described by this parameter.  If the C3D file contains more trajectories (read the parameter POINT:USED to determine the actual number stored in the 3D/analog data section) than are described by POINT:LABELS parameters then the additional trajectories must be referenced by number.

Note that while the POINT:LABELS are typically four upper case characters, some applications may create labels that are larger.  It is recommended that the POINT:LABELS values are consistent within a set of data files.