When you click M4A: AAC in the Destination area of the Bounce dialog, the following options are available:
Encoding pop-up menu: Choose Advanced Audio Codec (AAC) or Apple Lossless to set the encoding for your .m4a (commonly referred to as MP4) file. While both encoding algorithms provide high-quality audio, the AAC format uses a higher compression ratio, resulting in smaller file sizes.
Choosing the M4A: AAC option in the Destination area disables PCM > Surround. Split Stereo is still available.
If the selected sample rate is higher than 48 kHz, a copy of the bounced PCM file is created (with an automatic sample rate conversion to 48 kHz) before encoding. This conversion takes place because the M4A format does not support sampling rates higher than 48 kHz.
Bit Rate pop-up menu: Choose a rate between 16 kbps and 320 kbps.
Note: This parameter is only available when the AAC codec is chosen.
“Encode with variable bit rate (VBR)” checkbox: Variable Bit Rate encoding compresses simpler passages more heavily than passages that are (more) harmonically rich, generally resulting in better quality files.
Because not all media players can accurately decode VBR-encoded files, this option is off by default. If you know that your target listeners can decode VBR-encoded files, you can turn this option on.