Never knew this. I guess we should start referring to it that way, but I doubt it'd catch on.
The main reason it almost never gets used is that it's almost never actually useful or relevant. You'd basically never hear of 2PP, because...that would be purely in-house, yet still a product of some kind? As soon as it starts being proposed to
other users, it's automatically either a third-party product if it's still an add-on for the original first-party product, or it becomes its own first-party product.
E.g. while it was in playtesting, arguably, Pathfinder was a second-party product
for Paizo itself, because it was product created "for" 3.5e, but only to be used internally by Paizo. But as soon as Paizo started selling its Pathfinder system on its own merits, it became a new and distinct first-party product based on the same fundamentals.
It's a bit like how there are lots of first-person games or narratives, and lots of
third-person games or narratives, but very few
second-person games or narratives. A second-person novel would be incredibly difficult to write and a really weird read. A second-person video game is...well I'm not even sure if it's possible, but I suppose something along the lines of
Black & White might qualify, where the actual "participant" is not the player, but an artificial intelligence being given non-binding
instructions by the player. (Similarly, "second-person perspective" is basically never used, because that would mean basically being at the level of having a
conversation with the player character, which would be...really really difficult to employ in a gaming context.
So yeah.
Technically "second-party material" does have a meaning, but it's so incredibly narrow and uncommon that there's not much need. Particularly since the vast majority of (actually-used) "2PP" is simply DM homebrew/house-rules/improvisation and thus not really a "product" proper. 2PP does sometimes actually come up in a software-design context, though, if a licensee of a particular product makes their own software for internal use that depends on the licensed product. For example, you license the Havok physics engine and then make a software module that builds off of it to provide some other useful functionality. Perhaps, for example, you build a cinematic-recording suite that can be used in any of the games your company makes as long as they're using the Havok engine; that recording suite would be second-party product as long as it stays within the company. It would become a third-party product (relative to the Havok engine) if you sold that recording suite to other people.
