RejectedSoftware Forums

I just stumbled upon this:

{
    "systemDependencies": "libevent 2.0.x or libev, OpenSSL 0.9.x or 1.0.x",
}

in the vibe.d package.json:
https://github.com/rejectedsoftware/vibe.d/blob/master/package.json#L14

but i cannot seem to find a specification for it here:
http://code.dlang.org/package-format

what does it do ? or is it just documentation ?