includeLfe

Fold the low-frequency effects channel into the stereo mix.

False by default, which drops it. This is not a preference, it is a measurement: a 5.1 clip carrying a 60 Hz tone in its LFE and silence in every other speaker comes out of ffmpeg -ac 2 as EXACT silence, and ReferencePcmTest pins that. ITU-R BS.775 says the same thing: the LFE is a separate effects feed for a subwoofer and not a bass instrument, and summing it into two full-range speakers makes film audio boom and eats the headroom the dialogue needs.

The engine used to fold it in at -3 dB, and the surround fixtures kept their LFE silent so that the disagreement with FFmpeg never showed up in a test.

True folds it in, at -3 dB into each front speaker or at unity into a lone centre, which is FFmpeg with an LFE level of 1. That is for a caller who would rather keep the content than match the reference.