This is the number, during the relevant period, of days of treatment. 

For the purpose of recording and reporting days an insurer should refer to the National Minimum Data Set (NMDS). A day is measured from midnight to 2359 hours. 
When reporting by age category, the number of days should be reported against the relevant age at the time the treatment was done, not the against the age of the person in the current reporting period, because sometimes claims are made and benefits paid long after the actual treatment.