The BLT Sando is the Japanese reading of the American bacon, lettuce, and tomato sandwich, and the word "reading" is doing real work, because the Japanese version is recognizably its own thing. The three named components are all present, but the proportions and textures shift toward a softer, creamier, gentler sandwich than the crisp, salt-forward American original. That shift is the entire subject of this entry.
The differences live in three places: the bacon, the bread, and the binder. Japanese versions often use a softer, less aggressively rendered bacon, sometimes a thicker, hammier cut that stays supple rather than the brittle near-shattering rasher of an American diner BLT. The bread tends to be soft white shokupan, frequently with the crusts trimmed and cut into clean rectangles or triangles, rather than toasted country bread, so the whole sandwich is pillowy rather than crunchy. And the mayonnaise is applied with a heavier and more confident hand, a Japanese egg-rich mayonnaise with its tangy sweetness running through every bite rather than a thin schmear. The lettuce is usually crisp shredded or a tidy folded leaf, the tomato thin and de-seeded enough not to flood the bread. A good Japanese BLT is a balanced, creamy, savory thing where the bacon's smoke reads through the mayonnaise; a weak one is a wet wrapper, pale flavorless bacon, and tomato turning the crumb to paste.
Variations stay within that softer frame. Avocado is a frequent fourth letter, slipped in for richness and a green stripe in the cross-section. A fried or sliced boiled egg turns it toward a fuller breakfast sando. Some versions toast the shokupan lightly to recover a little crunch without abandoning the soft style, and convenience-store builds tune the whole thing for a clean cold cross-section behind plastic. The American toasted BLT on crusty bread, with its emphasis on shattering bacon and a far lighter hand with mayonnaise, is a genuinely different sandwich and deserves its own article rather than being crowded in here.