You have an 8" Clean out because you have an 8" sewer line.
I don't know how big your building is, or what water and sewer services you are utilizing, but I can tell you that you should be very grateful to have a large sewer line. All too often we run into issues where the sewer is too small for the proposed fixture units, and it becomes a big pain in the ass to fix and a big unwelcome expense for the building owner to repair.
Don't worry... Be happy. Seek out the parts for your 8" claen out with a smile on your face