I mean, I'm still pretty shocked to hear 'motherboard' isn't allowed in external product documentation. I don't think it matters if it's internal or external.
"Mainboard" is arguably a bit more literally descriptive: it is the main board of the device.
The figurative part of "motherboard" is pretty vague: it's just larger than the "daughterboards" and in charge of them--it doesn't birth or nurture them or do anything that's stereotypically maternal.
Often power is transmitted from the motherboard to be daughterboards, and the two are connected via a conduit like a fetus in the womb. It's not a completely arbitrary metaphor.
As for whether it's the main board or not, surely that's a matter of opinion. An AI researcher would be much more interested in what the GPU board is doing than on what the motherboard is doing.
It's certainly more central to the device itself. You can run a computer without a GPU, NIC, sound card, etc, but regardless of the peripherals you want, you're gonna need some kind of motherboard/mainboard to tie everything together.
I've heard claims that it's a motherboard because it has female connectors (i.e., sockets), but that's sort hard to square with the contemporary coinage of "daughterboards."
