Doctor Q said:If you mean ?, it's the symbol for the option key. Why? I've always wondered. It doesn't look like an option to me. On the other hand, what does an option look like? Maybe the symbol [] could have been used, since square brackets often mean optional in extended BNF and other syntax notations. But it's no use second guessing. Apple chose ? and that's what we get.
A thought immediately came to my mind regarding that symbol when I read this post. Isn't it obvious that the symbol represents an optional path? The horizontal line represent one choice and it is broken, meaning that you chose an optional path or choice.