These are just some of my observations as someone learning Houdini's nodes and codes.

The attribute wrangle (geometry) supports the access to the wire inputs via the “OpInput#_” to allow access to the node inputs (1,2,3,4). Incidentally “Bound Variables” in the Volume Wrangle specifically mention this access and does a nice job of explaining how to use the SOP.

The help file states.
“A number of variables are bound in the Volume VOP context. Use the @ prefix syntax to access them.

Time increment between frames, in seconds.
OpInput1, OpInput2, OpInput3, OpInput4”

I am pretty sure all wrangles work this way but the only mention of this access in the documentation on the attribute wrangle is when it mentions opinput#_ on the bindings section… but this is actually unrelated because OpInput is the contextually correct usage for vex snippets. Also in the terminal vcc -X does not list this as being available and its also nowhere in there other parts of the help file. I really wish there was a bit more consistency within the help file. I really feel like it could use some updating. Of all the wrangle attributes the Volume Attribute SOP seems to have the most through documentation.