D-Link DCS-6513 User Manual page 59

Section 3: Configuration
Size of each file for recording:
If this is selected, files will be separated based on the file
size you specify.
Time of each file for recording:
If this is selected, files will be separated based on the
maximum length you specify.
File Name Prefix:
The prefix name will be added on the file name of the
recording file(s).
D-Link DCS-6513 User Manual 59