Field help (DGGCOE) 

The density is a coefficient used to calculate the net quantity of hazardous materials of a product container: Hazardous material qty = (gross weight product container - tare product container)/density

The density can be modified for "liquid" products only (they are products characterized by their referred unit of "Volume" type).

On the other hand, the density cannot be modified and it is equal to "1" by default for the "solid" products (they are products characterized by their referred unit of "Mass/Weight" type).

Note: This setup belongs to the hazardous material functionality.