For example if you work in an in-patient hospital setting, you may wish to record the names of clients/patients in your ward at midnight.
This will help you keep track of the clients/patients you have included in the prevalence over a 24 hour period. You would not include any new clients/patients who were admitted during this time. By recording the names of individuals at the start of the count, you minimises the chance of double counting during the prevalence data collection period.