[FFmpeg-user] Need more detailed description of the behaviour of the "-ac <channelcount>" option

[Christoph Gerstbauer]

Hello

I need to know how -ac <channelcount> is exactly working.

In the documenetation following is mentioned:
-ac[:stream_specifier]channels(/input/output,per-stream/)

    "Set the number of audio channels. For output streams it is set by
    default to the number of input audio channels. For input streams
    this option only makes sense for audio grabbing devices and raw
    demuxers and is mapped to the corresponding demuxer options."

But there is nothing descriped for these usecases:

Inputfile has 2 channels (1stream) -> outputfile setting -ac 8 
(8channels, 1 stream)
Inputfile has 8 channels (1stream) -> outputfile setting -ac 2 
(2channels, 1 stream)

I tested a Stereo file to encode into IMX50 with 16bit audio an 8ch.
The stereo pair was correctly placed at ch1 and ch2. The other 6 
channels were silent (correct).
Is this behaviour standard for -ac ?
Or is there any mixing made in some cases? (8ch -> 2ch?)

I just want to prevent surprises when using -ac ;)
So is -ac just
+ cutting of or
+ adding silent channels?

Best Regards
Christoph Gertsbauer