That string is a standard file naming convention used by digital media release groups to identify a specific version of a movie.
: This is likely the beginning of the "release group" tag (e.g., AMIABLE, ARCHiViST), which identifies the community or individual responsible for creating and uploading that specific digital copy. Watchmen.2009.Ultimate.Cut.1080p.BluRay.H264.AA...
: The original source material used for the digital encode was a physical Blu-ray disc. That string is a standard file naming convention
: The movie title and its original theatrical release year. Watchmen.2009.Ultimate.Cut.1080p.BluRay.H264.AA...