This is the unique identification number for that particular video.
File names using this format are common for digital media listings on Japanese content marketplaces. The alphanumeric prefix usually categorizes the type of distribution, while the numerical string serves as a reference code within a database to distinguish one entry from another. FC2PPV-3161981.mp4
Based on the text , this refers to a specific video file hosted on the FC2 Content Market, a Japanese platform that sells digital content. FC2PPV: Indicates it is a Pay-Per-View video. This is the unique identification number for that