Anyone know the link between Asteroid and Extractor placement?

I still can't seem to find the link between the Resource Asteroid and the Extractor that the game places on it. How does the game know to put the extractor on the Asteroid and not let you place it anywhere in the gravity well like other structures?

I've looked through the various extractor entity file and the resource asteroid mesh files, the latter are in text format so I can't import them into max to look for any dummy objects but within the file there is this:

    NumPoints 1
    Point
        DataString "Extractor"
        Position [ 0.232924 377.611206 -12.912205 ]
        Orientation
             [ 1.000000 0.000000 0.000000 ]
             [ 0.000000 1.000000 0.000000 ]
             [ 0.000000 0.000000 1.000000 ]

I'm guessing that marks the asteroid for where the extractor mesh will be placed but I can't find and reference to it within the extractor files.

Does anyone know what I'm missing or is it part of the game we can't touch yet?