If I add a geo say box.
Go to Create Asset (or after the operator Type Properties)
and add (min/max) Inputs that then appear on the node level for connections.
Then in the Input/Output TAB you can set the Input 2 Label to say “extra” (#1 is parent).

How when I'm inside the node to I access the input (say another geo second input)?

If it's a SubNetwork node (with four inputs along the top) you see the 4 little square “Sub-Network Input #?”s inside it.
I expect(ed?) those to show automatically when I create them in the operator type panel?

Now I'm also pondering about what a “Spare” is and if that's preferable/applicable here?
Still I would like to make a connection inside from those connection inputs you see along the top of the node in the NetworkView.

Seems like a pretty advanced topic but kind of critical to a beginner H/coder too.

That's why I expected it to be part of the “Add” menu you get at the top of the opposite-click popup menu (when inside the sop node), or maybe I'm thinking a Parameter options setting (when outside).
Something like “Show Parm in:” “as Node Input”

There's actually a number of places that would be logical and useful to simplify this idea/dream (aka well hidden secret).
inside “Node Connections> Show external inputs”
or outside “Add Sub-net input”
maybe it's stuff for Spares?
How about TAB (add node type) “Sub-Network Input”?
etc. still I've hunted for hours and found nothing anywhere to help.

when you create an Object (e.g. Box as you said)
and convert it to HDA
then your HDA will be Object Geometry Operator in other words Object which holds Geometry (SOPs) inside

in that case you cannot pass Object inputs to inside SOPs because they are not compatible
you can access the data from such inputs through expression functions in combination with opinputpath() and use them however you want

if you create your HDA from Object Subnet or create new one of that type, then you will have all inputs passed inside as you expect, since such Object holds Objects inside as well (the same for any subnet HDA)

“Spare” Parameters are parameters added to an instance of any HDA/Operator, so they are not part of Operator Definition
they are usually added through Cog menu
or did you mean “Spare” in other sense? it isn't clear for me what you meant

Thanks for the feedback.
I guess I've also come to a similar conclusion via other routes over the last few days. So there is not a way to do it ‘as expected’.

I suppose another point to make is that if you select a couple of primitive nodes on a scene level (or deeper) and opposite-click empty space for the menu and select “Collapse” in will make a Sub-Network without having to cut&dive&paste etc.

Still in general as one new to Houdini the concept of a “connection manager” seems to be possibly forgotten. Typically in other 3D apps when you draw a connection between nodes you can see a dual popup menu with the ability to see all the possible outputs and which possible inputs can be connected (as you pick a give output).
There should be a way when inside a node to see (and connect) the ‘globals’ (for lack of a better relevant term) .
These other systems also typically include colored lines and other interesting ways to show the varied connection paths and types etc.

In Houdini I'm seeing something about “Promoting” properties in VEX, but that seems to be also missing from the Geometry levels (as a possible alternative solution to my quandary).

Also there are no array of outputs etc. I think you can see from what I'm saying it does appear to be limited in the expected node connection GUI, that maybe I've only recently seen touched on in VEX.

You guessed right with the reference to Spare I think I meant Spare Parameter, which is I believe associated with “Operator Types” and/or “(Houdini) Digital Assets” or HDA as you put it. Still I don't think it's making connection inputs on the nodes.
In the end I guess it's been decided that that way of thinking is reserved/restricted to VEX, so maybe that aspect is something I as a developer would have brought to question long ago.

GottaGetmedat
Still in general as one new to Houdini the concept of a “connection manager” seems to be possibly forgotten. Typically in other 3D apps when you draw a connection between nodes you can see a dual popup menu with the ability to see all the possible outputs and which possible inputs can be connected (as you pick a give output).
There should be a way when inside a node to see (and connect) the ‘globals’ (for lack of a better relevant term) .
These other systems also typically include colored lines and other interesting ways to show the varied connection paths and types etc.
.

I am sorry but I shudder at the thought of having a maya type based connection manager in Houdini.The otl system based on a tool living in a context is perfect.