The American Dental Association (ADA) is very specific regarding classification of codes, per this list which is derived from The ADA Practical Guide to Dental Procedure Codes
PREVENTIVE
· Diagnostic
o D0100 through D0999
· Preventive
o D1000 through D1999
BASIC
· Restorative
o D2000 through D2999
· Endodontics
o D3000 through D3999
· Periodontics
o D4000 through D4999
· Oral and Maxillofacial Surgery
o D7000 through D7999
MAJOR
· Prosthodontics (Removable)
o D5000 through D5899
· Prosthodontics (Maxillofacial)
o D5900 through D5999
· Implant Services
o D6000 through D6999
· Prosthodontics (Fixed)
o D7000 through D7999
ORTHODONTICS
· Orthodontics
o D8000 through D8999
OTHER
· Adjunct General Services
o D9000 through D9999