Meet management made simple.
Classifications are a separate way to group athletes. Where as Groups have a one-to-one relationship (an athlete can only be in one group), an athlete may be in more than one classification.
This concept was initially created for Paralympic Meets that have different disability classifications. For their purposes a classification may be set as Track, Field, or Either... because different disability classes impair different events.
However, classifications can be used for other things. For example in a road race you may have athletes put into age groups, but you also want to be able to separate results out into other groups: a Clydesdale division, Local runners, Wheelchair, Nude vs. Clothed (in certain nudist resort races), Costumed runners for Halloween races, or some other goofy and fun things that road races might do.