Linear PCM vs. Bitstream audio

I have my PS3 hooked up to my Sony 5.1 Surround Sound System using an optical cable. Under the BD Audio Output Format you can choose between either Linear PCM and Bitstream. What is the difference between the two? If I select Bitstream the PS3 tells me that some portions of audio from the BD may not be played. What exatly does that mean?

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