What exactly does door B mean in photography? Want the most specific

door b means closed door! ! ! This is about the shutter time and the light advance! For example, it takes 1/8 shutter in general light and 1 to 2 seconds in poor light! And in the case of serious lack of luminosity, in order to ensure sufficient light, use B door! Turn to door b and keep pressing it, and press it for a few seconds to enter the light for a few seconds! This applies to film machines without flash! It seems that all the digital ones have flashes.