detections numpy array（fieldNames = ['frame','id','x','y','w','h',score]）, What does ‘id’ mean？

[sunaixiadashuaibi]

detections path to csv file containing the detections or numpy array containing them.
Loads detections stored in a mot-challenge like formatted CSV or numpy array containing them.

numpy array（fieldNames = ['frame','id','x','y','w','h','score']）, What does ‘id’ mean？

Thank you for your reply