In the words of Source Audio:

The Atlas Compressor offers a comprehensive collection of compression circuits that perfectly capture the tone and feel of the industrys most sought-after compressors. Straight out of the box, Atlas delivers six different styles of compression, ranging from classic stomp-box compressors like the now-discontinued Diamond Optical Compressor to high-end rack units such as the snappy 1176 compressor and the smooth and gentle LA2A optical compressor.