Copy on Access resolve the problem in the scnario when data is copying during TF/Clone is active, but Host need to access the track which has not been copied. Copy on Access will copy the track first to the target  Precopy enables the background copy during TF/Clone creation
what is the difference between  between  copy on access and precopy with differential in timefinder clone
difference between copy on access and precopy with differential in timefinder 
clone