A topcoat is the lighter of the two coats, usually in a medium-weight wool somewhere around 300 to 400 gsm, and it works as an outer layer through the cool weeks and on trips to temperate places. In Chennai this is almost certainly the coat you want, not a full overcoat. It handles our short winter, and it handles Singapore, Delhi in December or London in spring without you sweating through the terminal. We make it exactly as we make everything else: a pattern from your measurements, cloth off our own shelves, and a fitting before it goes out.
Which of the two you actually need
A topcoat is lighter, a shade less formal and much more useful day to day. It goes over a suit jacket without adding bulk across the shoulders, which makes it practical for ordinary working use in cool weather. An overcoat is heavier and dressier, and it belongs to real cold and to formal occasions. Owning the wrong one of the two is a common and expensive mistake.
If you are based here and travel to cooler places a few times a year, a topcoat in a medium worsted or a fine wool-cotton mix is the better buy by a distance. It packs down far more easily than a heavy overcoat and it stays wearable across a much wider range of temperatures, which matters when you land somewhere twelve degrees colder than the city you left.