This is an inverted triangle, with its sharp corners facing down and red. This emoticon is commonly used in elevators or other machinery, and is used as a "down button", which means that the elevator goes down and the machinery goes down.
The icons depicted by different platforms are different. In terms of color, except for the orange button depicted by the au by KDDI platform, the buttons depicted by other platforms are all red; In terms of shape, most platforms depict equilateral triangles, while Messenger platforms depict isosceles triangles with relatively long lines below. In addition, Microsoft and OpenMoji platforms depict black edges on the periphery of the diamond. As for au by KDDI and Docomo platforms, a white line is added to the right side of the triangle to represent the luster of the graphics.
