BTW, usually select all buttons have three states - none, partial and all. When the user selects or deselects some of the options, the button goes into partial mode and that way it can also be used for deselecting all the options.
BTW, usually select all buttons have three states - none, partial and all. When the user selects or deselects some of the options, the button goes into partial mode and that way it can also be used for deselecting all the options.