It's a calculator predominantly. It can help you calculate AOB when you know your own course, their course and true or relative bearing. The other side can help to calculate the AOB based on other factors. If you've got a formula for it that depends on multiplication and division (and maybe some adding/subtracting done on paper) then you can use it for that. For example, it is possible to calculate AOB based on apparent sizes measured with the periscope scales and the 'real' sizes in the recognition manual. Or if you established constant bearing situation and know their exact speed (and ofcourse know the bearing and own speed) then aligning the scales in a certain way you can come up with the AOB. But aside from the visual ship rotator function it does not tell you otherwise what the AOB is. It is a general calculator, the intelligence must come from you. It doesn't care how you perform them. Something comes out whether it makes sense or garbage. And not just AOB. Though there are some procedures shown in the accompanying videos (narrated by derstosstrupp).
In the download section there is also the Angriffscheibe Manual (Attackdisk Manual) which may contain other procedures not mentioned in the videos (and vice versa) that can be applied to it. It's made for a different implementation of the 'non-historical' sliderule side. (templates built to make physical DIY or modded implementations in game.) But the manual should provide a good introduction on how to use it before you buy the Sub-buddy app for real.
[Edit]: I guess this is advice 5 minutes too late.
