dub configuration for a dependency?

[joshuabarnes via Digitalmars-d]

By default, DerelictODE is configured to load the 
double-precision version of ODE. If you want to load the 
single-precision version, declare "DerelictODE_Single" as a 
version in the build settings of your dub.json.

What does this actually mean?
I have

    "dependencies": {
        "derelict-glfw3": "~>4.0.0-beta.1",
        "derelict-gl3": "~>2.0.0-beta.7",
        "imageformats": "~>7.0.0",
        "derelict-ode:": "~>3.0.0-b"
    }
but have no idea what the above means? sadly the dub 
documentation didn't seem to be much help?