The trap needle metering system means that the meter can only function in auto mode.
However, the display in the viewfinder gives the photographer the ability to read the indication, and then switch off the meter and set the camera to the appropriate manual setting, based on judgment and what the meter viewfinder indicator suggested.
It is sort of a built in hand meter - more convenient than the meters on the top of the camera, that required one to move one's eye from the viewfinder to read the light.