channelLayoutMask

Which speaker each channel drives, as the native order mask: one bit per speaker, channels in the order of those bits from the lowest upward.

This is what the engine's downmix keys on, and channels cannot replace it. Six channels are 5.1 with side surrounds or 5.1 with back surrounds, and a downmix that guesses wrong sends the surround content to speakers the mix never intended. Null when the source declared no layout, or declared one no mask can describe, which is a custom channel order or ambisonics. The engine then falls back to the channel count and says so through a warning.