To summarise the definitions from the guidelines: 

•  Clostridioides difficile infection (CDI) :  Clinical signs consistent with CDI and microbiological confirmation of C. difficile toxin presence, indicated by either an enzyme immunoassay showing free toxins or a positive nucleic acid amplification test (NAAT) with a low cycle threshold (Ct) value.  Additionally, a toxigenic culture or a diagnosis of pseudomembranous colitis found during endoscopy, colectomy, or autopsy alongside a positive toxigenic C. difficile test can confirm CDI.
• Diarrhea in CDI is defined as three or more loose stools within 24 hours, consistent with a Bristol stool scale of 6–7.
• Severe C. difficile infection: White blood cell (WBC) >15,000
  cells/mm3 or serum creatinine .1.5 mg/dL.
• fulminant infection as patients meeting criteria for severe C. difficile infection plus presence of hypotension or shock or ileus or megacolon.

• Treatment response is marked by diarrhoea resolution, with the patient producing formed or normal stools for their baseline, maintaining this improvement throughout therapy and for at least 48 hours post-treatment. Successful treatment also requires improvement in clinical, laboratory, or radiological parameters without new signs of severe disease

• Treatment failure is defined by lack of symptom resolution, worsening, or new signs of severity.  A notable reduction in bowel movement frequency may suggest a partial response. Treatment effectiveness is typically monitored daily and evaluated after three days unless symptoms worsen. Treatment with metronidazole may yield a response only after 3–5 days, and stool normalization may take several weeks post-respons

• Refractory CDI is defined as CDI that does not respond to recommended antibiotic treatment after 3–5 days. Refractory CDI may occur in both non-complicated and complicated cases

• Recurrence is diagnosed if CDI symptoms reappear within eight weeks of a prior resolved episode, though studies differ in the follow-up period, with some using 4 or 12 weeks. Distinguishing between recurrence due to relapse  from remaining infection is difficult and only can be proved with genetic test of the bacteria strain, which are not usually performend.

Remember than shedding of C. difficile can be seen in up to  56% of patients who had resolution of diarrhea as long as 4 weeks after completing treatment, therefore stool retesting is not useful to determine infection resolution, only clinical symptoms determine if infection is solved.