Usually 0 or 1, used to determine the direction of the transfer. Carries either a fixed or a programmable data. Classifies the transfer. Generally, USB device can use any field for any purpose, but these cameras appear to follow the general idea of using index field for directing the value to appropriate receiver. Those are fairly low-level transfers.
Camera uses several of such transfers grouped together in packets.
One packet usually carries out one high-level adjustment. Model 4 has some packet structure too, but I can't figure it out. Looks like each packet has variable format and variable length, and the framing of packets in this camera is very fluid, defying analysis. Therefore I only separated one packet that is known to adjust gain in color channels and affect hue, in some way . Some Model 3 cameras are equipped with an RCA connector which can be used to feed a standard video NTSC signal into the camera.