Neutral mesh format specification

I would like to understand how the neutral mesh format works, but I can’t find a filetype specification in the documentation or anywhere else. Does anyone know where I can find this info?

From reading the files directly, I can infer the general structure, but I’m not sure what some things mean. This is what I’ve figured out so far:

<# of points>
<x> <y> <z>
<# of volume elements>
<???region???> <point 1> <point 2> <point 3> <point 4>
<# of surface elements>
<???> <point 1> <point 2> <point 3>

Any help is much appreciated.