Is "density per unit volume" something? I was reading articles on the preparation of shellac-based composites when I encountered the following statement (in ref. 1): 


  Wood flour and fibres are interesting because of their low cost, good specific  strength, low  density  per  unit  volume,  along  with  their  renewable  and degradable  features. The  drawbacks of  natural  organic  fillers include poor  adhesion between the filler and the polymer matrix, and poor thermal and water resistance (Saba et al. 2014).


Note the same phrase is used in the reference cited in the passage quoted above (Saba et al. 2014) but then the trail goes dead. 

Is "density  per  unit  volume" a typo or other error that's been propagated during citation or a real concept of concern for material scientists? If it is, what is the meaning of this concept?  

References


Obradovic et al. (2017). “Cellulose-shellac composites,”BioResources12(1), 1943-1959.
Density usually specifies a ratio (or equivalent derivative) of the type "amount/unit dimension" where dimension is usually one of (distance, area, volume, hypervolume). Based on further examples of usage (see links in Todd Minehardt's answer) of the expression "density per unit dimension" where "dimension" might be "distance", "area", "volume" or other spatial dimension, the point with writing "density per unit volume" seems to be an attempt to be explicit about the dimension in the divisor (denominator). For instance, when writing  "density per unit dimension"  it is meant that "density=amount/dimension". Thus "density per unit volume" means "density=amount/volume". One could equally well say "volume density" or "area density" to be specific, but there might be ambiguity there as well, as one might confuse usage with expressions such as "number density" or "energy density" where the property in the numerator is being specified. Then "volume density"  might be interpreted as meaning "density of volume", even if farfetched. One could specify dimensions by placing the units in parentheses, for instance, as density (per unit volume) or density (per unit area) etc.