And, to give a very brief summary of what the ratings mean...
The letter part of the grade is the "coverage". Out of all the possible tags and tag combinations, how many of them have at least one pic? Fixations in general only need one pic for this, but farm pics want there to be one for each sex act for each farm style. Holes in coverage do not mean that no pic will be available, but rather that the game will use a ranking of tags to try to figure out a next best pic.
As an example, if the game wants a "dildo" pic, and there is none, it'll then look for another "toy" pic to use instead. If no toy pics exist, it moves up the chain to use a "profile" pic. The profile pics pull extra duty as pics of last resort for most any other missing tag.
The +/- part is "depth". As an average, over all the tags, of how much variety there is for each tag. In this case more variety, more pics for each tag, will mean a better +/- score shown.