The big picture tag is for a "broad, overall view or perspective of an issue or problem."
big-picture tag is for questions that have a larger scope. They might have a very specific answer (and so would not automatically require a CW tag), but they are more philosophical in nature. One good example of a big-picture question is
because it's asking a specific question ("what is the meaning of a semantic complexity class"), but it's also exploring the deeper meaning behind the intent of such classes.