Those are internal APIs - interfaces for writing virtual devices, such as motherboards, chipsets, video cards and so on.

Is there anything specific you would want to implement?

Sorry, but I guess that means internal to 86Box, and if so what is driving them, or where would you interact with them?

I would like to be able to add a PCMCIA card that is not a Flash memory card, or anything else that's commonly in the VMN settings GUI.

Yes, I mean internal to 86Box - virtual devices are typically C modules, linked into the main binary. The interaction happens through function calls and global variable manipulation. Plumbing happens in src/device.c
There are no dynamic loadable module system or interprocess communication protocols to implement a virtual device.
To get a basic understanding, start reading an init function of a machine - it adds the devices that the machine is built of.
Currently I don't see any code that would implement a PC Card host interface, so that won't be an easy task.

I was reasonably certain I was out of my depth, many thanks for your confirmation.

Problem I have is that this card is a link in a chain from a W95 piece of software, to a complex electrical box running a machine usefully. Currently keeping a stock of period laptops and disk images available, for when the current one dies.
We'd all feel better if we could host it on something current and a VM looked very promising and works well with recognised hardware like NIC and COM ports.
The sponsors would be happy to buy a 'box' that the PCMCIA card plugged into, with maybe a Thunderbolt or USB link to the new PC, but I've no idea who to approach with a view to rewriting the W95 PCMCIA driver to successfully communicate with my imaginary interface, assuming it is possible. I guess somethings cannot be achieved for sound reasons.