How does out work ?

[stuaxo]

I'm trying to get some DOS mouse code working, but can't work out how out() works from the README, I'm translating this code, that outputs into bx (button pressed), cx, dx (x, y)

void get_mouse_info(int *xpos, int *ypos, int *click)
{
    in.x.ax = 3;
    int86(MOUSE_INTERRUPT, &in, &out);
   *click = out.x.bx;
   *xpos = out.x.cx;
   *ypos = out.x.dx;
}