The following classification is used in general surgery:

Hinchey Classification of Diverticulitis

Stage 1a: Phlegmon (inflammation)
Stage 1b: Diverticulitis + pericolic or mesenteric abscess
Stage 2: Diverticulitis + pelvic abscess (abscess confined to the pelvis)
Stage 3: Diverticulitis + purulent peritonitis (peritonitis not involving the bowel lumen)
Stage 4: Diverticulitis + fecal peritonitis (peritonitis involving the bowel lumen)