AAC (short for Advanced Audio Coding) is a compressed audio format similar to MP3, but with extra performance optimizations.
Unlike MP3, the AAC format can preserve the initial audio quality for the encoded audio track. Also, AAC has higher coding efficiency and it also handles frequencies above 16 kHz much better than the MP3 format can.
The usualy way of creating a custom AAC file is to convert any (audio or video) file to this particular format using dedicated apps.
In order to make sure that the generated AAC audio file will meets the users' necessities, one will also be able to adjust the bit rate and sample rate of the source audio file.
One will also have the option to choose the destination folder and the output filename for the newly-created AAC file.