What is the function of the vertical interval signaling (VIS) code sent as part of an SSTV transmission?

There are many different SSTV modes (such as Scottie, Martin, and Robot), each with unique timing, resolution, and color encoding standards. The Vertical Interval Signaling (VIS) code is a short digital header sent at the very beginning of an SSTV transmission. Its primary function is to automatically tell the receiving software exactly which SSTV mode is about to be transmitted. This allows the receiving software to automatically switch to the correct decoding parameters without the user needing to manually select the mode. Distractors suggesting it provides vertical synchronization or identifies the call sign are incorrect. While "vertical interval" sounds like a sync pulse from old analog broadcast TV, in SSTV it strictly acts as a mode identifier.