An output ID is included in the format ID and starts with a
# (think CSS ID).
You must set an output ID if you create 2 or more outputs with the same signature.
For example, if you wanted to create one regular and one squared thumbnail with the same size, you would do it like this:
-> jpg:400x#regular = $s3/thumbs/thumb_#num#.jpg -> jpg:400x#square = $s3/thumbs/thumb_square_#num#.jpg, square=true