This is a long time exposure setting – shutter stays open for as long as you keep the shutter release button held down. Time exposure mode. Similar to bulb mode, only the photographer presses the shutter release once to open the shutter and once again to close it. Largely superseded by bulb mode on most cameras, for some reason. Odd, as I think T mode is more convenient to use than bulb, and no harder to implement on automated cameras. However, some cameras have a similar function with their electronic shutter releases, even though it isn’t called T mode. For example, Canon cameras which support the RC-1 infrared remote work like this in bulb mode. One press of the RC-1 shutter release opens the shutter; another press closes it.