There are three possible notches on super 8 cartridges. The 'centring notch' the 'speed notch' and the 'filter notch'.
In the centre of the cart, opposite the film slot is, you guessed it, the film centring notch. All cameras have a centring pin inside the film compartment. This pin and notch just help register and secure the film in the camera.
Above the centering notch is the speed notch. This notch varies in size depending on the speed (asa rating) of the film. Upon the introduction of super 8 film, the Society for Motion Picture and Television Engineers (SMPTE) in their infinite wisdom laid down a set of specifications for the notching of super 8 cartridges. This specification is given in terms of tenths of an inch (which are approximately multiples of 2.5mm). The distance specified was not, however, the width of the notch, but rather the distance from the lower edge of the notch to the film centering pin. Why they expressed it in those terms we will never know. Furthermore, they did not specify notch dimensions for the entire range of possible asa speeds. Rather, they left a few out - seemingly at random. There is no specified notch length for 32, 50, 80, 125 or 200 asa. Oh well. Here is their list:
Daylight film speed - cartridge has no filter notch
Tungsten light film speed - cartridge has a filter notch
As can be seen, the distance from the centre pin to the bottom of the cartridge notch decreases as film speed increases. Thus, the size of the notch itself increases with film speed.
One thing that is confusing about the table above is that there are two columns for film speeds - daylight and tungsten - and that the same size notch is used for different film speeds, depending on whether the film is tungsten or daylight balanced. For example, both 160 asa tungsten and 100 asa daylight films have the same notch dimension. How can that work? The answer is the filter notch!
If present, the filter notch is located about 2cm below the centering notch. Tungsten balanced film has this notch, daylight balanced film does not. Now, all super 8 cameras have an internal colour correction filter for shooting tungsten balanced film in daylight. Using this filter, however, takes away a little bit of light that would otherwise get to the film. Specifically, they reduce the light by two-thirds of an f-stop. Thus, a 160 asa tungsten balanced film when used in daylight with the colour correction filter in place, is effectively becomes only a 100asa film. That's why the 160asa notch for tungsten film is the same size as the 100asa notch for daylight film. Really, the 0.5 inch notch dimension is the 100asa dimension.
So what does the filter notch actually do? Cameras that can read the filter notch do so via a small pin inside the film chamber. When using tungsten film - ie film with the filter notch - the pin is not pushed in by the cartridge. In this instance the internal filter is kept in place. When daylight film is inserted in such a camera, however, the pin is depressed (because there is no notch) and the filter is automatically removed. Thus in our example of 160 tungsten, 100 daylight film, when the tungsten film is in the camera, the filtern notch pin is not depressed by the cartridge, so the internal filter is kept in place. The film speed notch reader indicates a speed of 100 asa to the camera - which is the right speed for 160asa film when shooting in daylight. When the 100 daylight film is inserted, the filter notch pin is depressed and so the filter is automatically 'de-selected'. Again, the film speed notch reader will detect a speed of 100 asa - correct for this film.
So what happens with tungsten balanced film when you want to shoot under tungsten light? As usual with tungsten film, when the cartridge is inserted in the camera, the presense of the filter notch will mean that the internal filter is not 'de-selected'. To shoot under tungsten lights, the internal filter needs to be deselected another way. This will either be done by inserting a filter key or a filter screw. In either case, not only will doing so remove the filter, but the light meter will also be compensated by two thirds of a stop. Thus, the filter notch pin when depressed 'de-selects' the filter but does not adjust the asa, while the filter key or filter screw when inserted 'deselects' and also ajusts the asa.
Not all cameras however have a filter notch detector. Those that don't are all of the 40/160 asa type. Instead of a filter notch pin and filter keys or screws, these cameras use a simple switch to select or de-select the filter. While easy to operate, the absence of a filter notch pin reduces the flexibility of the camera. 40/160 cameras that have the pin can not only read 40 and 160 tungsten film, but can also read 25 and 100 daylight film, while those with just a filter switch can only read 40 and 160 film, tuntsten or daylight.