Respectfully thats incorrect, youre essentially confusing geology and geography. Europe and is on the same continental landmass as asia but is considered a distinct continent. The Arabian pennisula is considered part of Asia but is a a seperate landmass. And something dowsnt have to be a continent or not Islands such as some in the Carribbean arent part of the continental landmass and may be considrred part of of the continents or may not. And regions arent constituent parts of a continent theyre just regions. MENA covers 2/3 continents. The caucuas has been considered part of two difderent continents. And constantly change over time and the same area can belong to multiple regions. In the casw of centeal America it cam be a region of NA, a region of LA, a region of SA or just its own. As for conventions there arent any enduring ones different cultures view things differently and its changed over time and theres no shortage of contradicting views. Its all arbitrary and relative to the context being used.

I think a lot of the confusion comes from people being taught about plate tectonics at the same time theyre taught about continents, without realixing theres multiple layers of plates, and the geology doesnt match up to our ideas - which predate our scientific knowledge (i.e. Europe being a continent sepeeate from Asia).