As far as I understand it, there’s no actual “official” definition. Second-party can mean both:
From the main supplier but retaining a different supplier’s branding (as opposed to rebranding, which makes it first-party again even if the manufacturer doesn’t change).
OR
The buyer, analogous to the second-person point of view (“you”), so in that context, a second-party battery would be one you build yourself.
NeatNit@discuss.tchncs.de 5 hours ago
From my understanding, the first and second parties are the seller (Valve) and the buyer (you). They are the two who have a contract between them. A third party, then, is any party which isn’t signed on the contract. That is, an outsider to the contract.
Any other meaning of second party is probably a back-formation from a misunderstanding of the terms 1st and 3rd party.
Disclaimer: I literally have no idea what I’m talking about