Proposed descriptor developed by SMPTE Working Group P18.41. This proposed SMPTE recommended practice is intended to provide a method of coding video index information in which various picture and program-related source data can be carried in conjunction with the video signal. There are three classes of video index data based on type and use of the data. Class 1: Contains information that is required to know how to use the signal. Class 2: Contains heritage information for better usage of the signal. Class 3: Contains other information not required to know how to use the signal.