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It is not clear to me what the various profiles (AAC-stereo, FLAC, WDTV) in properties are for.

I have been working until now with the default profile. What are the others for and what are their differences from the default profile?

I am rather new to video handling so there is some knowledge gap.

P.S.: I tried twice to include an attachment and I got an error message that the "board attachment5 quota has been reached". The meaning of this is not clear to me. Can some one clarify this message?

Much of what you're asking about is covered in messages in the Advanced Usage section of the forum, including some descriptions of how each part works.

Some are pretty simple - the WDTV profile simply makes sure that any PGS subtitles are compressed with GZip, if they weren't that way on the BD. The others allow converting audio in one CODEC into another (usually FLAC), to make the MKV compatible with older audio systems.