Mouse Connectivity Projection Density Units

I would like to use the data from the injections in the mouse brain connectivity tool, but I am having a slight issue understanding it.

When viewing the data, I have the option to switch the data from “Projection Volume” (in mm3) to “Projection Density.”
I am having an issue determining what “Projection Density” really means. Would anyone be able to provide a description and units for that?


Take a look at these two resources