Subantarctica or the Subantarctic Islands are a chain of islands with, to be honest, a very loose definition. They are located north of Antarctica, very often close the Antarctic Convergence (thats the area where cold antarctic waters meet the warmer water bodies of the Atlantic/Pacific/Indian Ocean). They still have vegetation like Tussock Grass, shrubs sometimes even small trees, often they are glaciated in higher elevations. Islands like the Crozet Islands, Macquarie, or the Kerguelen are very typical (and even harder to go to than South Georiga). South Georgia is typical subantarctic regarding vegetation and wildlife, but it is already quite a bit south of the Convergence (now called polar front). Therefore it is sometimes considered an antarctic island.
A bit north of the convergence you can find far south of New Zealand islands like the Snares, Enderby or Auckland, which have quite a lot of trees, but are considered subantarctic as well. My times as a scientist are long over, therefore I prefere the simple definition. As long as their fauna and flora is mostly subantarctic and they are "close" to the convergence I prefer to call them subantarctic...
These islands have never been inhabited, some had whaling/sealing stations, and most of them have now permanent scientific research stations. The Falkland Islands are again a bit different, as they are far north of the Convergence and are inhabited since well before 1800. But their vegetation was subantarctic as well as their original fauna, but with some south american species like geese, vultures and song birds.
More about all that later......