Refers to the audio codec (Advanced Audio Coding) used for high-quality sound.
The specific version you mentioned—"UNRATED 1080p HEVC HDRip S01 Part 1 Web Series x265 AACmkv"—refers to high-definition digital rips found on various third-party file-sharing sites. Refers to the audio codec (Advanced Audio Coding)
Indicates that the content may contain mature scenes that were not edited for standard television ratings. Refers to the audio codec (Advanced Audio Coding)