Ad Hoc is a term used to refer to something "as necessary" or "when needed". Some examples of typical ad hoc assessments are listed below!
Ongoing Subject Logs
Medical History
Medical History is added dependent on the subject's account of their own health and their medical records. Each subject will have a varying set of conditions and medical events to account for. Hence, each line item is added ad hoc, or as necessary.
Concomitant Medications
Medications are typically logged in a one to one fashion per the Medical History. Just like medical history, the number of items logged is entirely dependent on the subject.
Adverse Events
I'm sure you're noticing the trend at this point, but an adverse event is only logged if the subject experiences an adverse event. Therefore, the log is kept separate from the visits, and is only logged if a subject experiences an adverse event.
As Needed Documentation
Unblinded or Unmasked Documentation
If the study requires blinded or masked staff and the blind is broken, documentation is then submitted via the provided ad hoc form.