It uses the same super confusing syntax that MOV/MP4 uses, and barely anyone properly understands or uses correctly. Its one of those cases were borrowing the spec from MP4 was probably not a good idea.
In the most generic case I would argue that fields are often stored in the correct order (top first), or even as one frame, and only the display order changes, so you would use 1 = TFF and 9 = BFF.
Most newer advanced video codecs will store the field order in the codec headers themself though, and this field is likely ignored by the majority of players.
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LAV Filters - open source ffmpeg based media splitter and decoders
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