This is a sequential catalog or volume number, used to distinguish this specific release from others within the same production line.
The .mp4 extension (MPEG-4 Part 14) is the most widely recognized digital multimedia container format globally. If you encounter a file labeled like "PPPD-515.mp4", it utilizes this specific framework: PPPD-515.mp4
If your goal is to review or discuss the content: This is a sequential catalog or volume number,
Instead of PPPD-515.mp4 , a well-named file might look like: 2023-10-15_TokyoTrip_ShinkansenRide.mp4 PPPD-515.mp4