The mdaw folders are created based on the 'dispersion rule' of the your FileStore (Administration » Providers » FileStore Info » Edit FileStore » Edit [default] Storage Rule). Basically this rule helps separate your content by storing them into multiple directories.
The default rule is $dRevClassID[-9:-6:0:b]/$dRevClassID[-6:-3:0:b], which basically takes two 3-digit substrings of dRevClassID and encodes it into a Base64 string.