Hello Aidan, although I haven't used this particular meter the principle with all Gossen meters with zone scales is similar!
Not sure how familiar you are with the Zone System of determining exposure and correct development but it is primarily used with film stock (usually black and white but can be used with colour film).
In a nutshell ...
Find an area in the scene you are photographing that is in shadow but still has visible detail. This area or "zone" equates to Zone 3 on the Scale - 0 being complete black and 10 being pure white. If you take a spot meter reading of this area/zone the meter reacts as if it is 18% grey (this equates to Zone 5). So in order to gauge correct exposure the arrow on the dial on the meter needs to be moved from Zone 5 to Zone 3 - effectively underexposing the film by 2 stops. This will give an exposure that shows detail in the shadow areas of the scene. To calculate the highlights the development time of the film can be altered to effectively move the highlights up or down the scale by increasing or decreasing the time in the developer.
PLEASE bear in mind that its been a few years since using this system so my explanation may be a bit rusty!!
