Actually, looking back, that was probably not something I should have said.
ROC and the PRC are each (in their own legal systems) the de jure government of the same (widely formally recognized, usually with the PRC formally recognized as the government) country, “China”.
“Taiwan” and “mainland China” are de facto countries, governed in fact by the ROC and PRC, and are often treated as such by countries that formally recognize one China with the PRC as its government.
> why use the qualifier for Taiwan?
Because in most legal systems that aren’t that of the ROC itself, the PRC is the recognized government of the single de jure country of “China” that subsumes both the de facto countries of mainland China and of Taiwan. ROC is, in those systems, only and exclusively the de facto government of the de facto country of Taiwan, and not the de jure government of anything, and Taiwan is de jure a province of China.
> Because in most legal systems that aren’t that of the ROC itself, the PRC is the recognized government of the single de jure country of “China” that subsumes both the de facto countries of mainland China and of Taiwan. ROC is, in those systems, only and exclusively the de facto government of the de facto country of Taiwan, and not the de jure government of anything, and Taiwan is de jure a province of China.
Which legal systems are you referring to in your last paragraph? The US, for example, does _not_ recognize Chinese claims to Taiwan. They acknowledge that China makes them and has agreed not to have official relations with the ROC, but they've never agreed with the claims nor have they ever made any statement to the effect that the ROC isn't a country nor that the ROC is not a legitimate government.
Frankly I suspect your confusion is a result of a misunderstanding of UN resolution 2758. That resolution said that the PRC gets the seat of "China" at the UN. It never said anything about what the borders of the PRC are. It also never said that Taiwan couldn't become a member as "Republic of China" either. (Of course, since China can block that action unilaterally as a member of the security counsel it's a non-starter.)
Anyway I may be wrong about your misconception, but then I don't know what you mean exactly. The US definitely does not consider Taiwan as a province of the PRC. I can't speak for other countries' legal systems, but at least I'm not familiar with many in Europe that have that legal designation. Sure most don't have relations with the ROC, but that's obviously not the same thing.
I still don't understand the point of the qualifier. I don't hear people refer to China as a de facto country so why use the qualifier for Taiwan?