The standard input range is 0 to 1, where 0 is completely transparent and 1 is fully opaque. In most cases you should use a value between 0 and 1, however, if necessary you can reduce the value down as far as -3, which will allow you to time the opacity animation differently than the 'Out’ animation. The lower the value, the quicker the transparency animation will be compared to the rest of the ‘Out’ animation. This can be helpful in harmonizing complex animations.