Here it gets tricky

What you actually see as class on the screen on the item details page is an almagation of >3 fields.

it goes classification,dewey,subclass

Looking in the database for the above book it has classification=J dewey=612.82 subclass=

Hence J612.82

So for something like J612.32aa You'd have classification=J dewey=612.32 subclass=aa

Yet, where should/could I put classification (J) into the database from your web interface? I did not find a field called classification or similar in koha web site. Please enlighten. Thanks a lot! Yaogeng