I have read in many posts that a locked bootloader prevents from flashing custom ROMs, because it checks the digital signature of the ROM itself.
I thought a "bootloader" was only responsible for loading an image at startup, thus a locked one would refuse to load an unsigned image.
Therefore, wouldn't it be more accurate to say that a locked bootloader refuses to load/boot an unsigned image, but it cannot do anything to prevent one from flashing that image ?
Of course flashing something one cannot boot does not make any sense, but that is not the point of my question. I just want to learn exactly what the locked bootloader on a phone does.