: A unique identification number, often the serial number of the recorder or a specific event ID.
: Stands for Sub-stream . Security systems record in two modes: Main-stream : High resolution for playback and storage. 38499-70 701531471561_Sub_f_1532161730.mp4
The filename indicates it is likely a video file recorded by a DVR or NVR security system (often associated with brands like Hikvision or Dahua ). Meaning of the Filename Parts: : A unique identification number, often the serial