Constant rsmpeg::ffi::AVIODataMarkerType_AVIO_DATA_MARKER_UNKNOWN

pub const AVIODataMarkerType_AVIO_DATA_MARKER_UNKNOWN: u32 = 3; // 0x0000_0003u32

This is any, unlabelled data. It can either be a muxer not marking any positions at all, it can be an actual boundary/sync point that the muxer chooses not to mark, or a later part of a packet/fragment that is cut into multiple write callbacks due to limited IO buffer size.