What does hash (#) sign do outside loops in SASS?

Question

I've just come across the use of the hash sign outside of a loop in sass and I'm not sure what it's used for or what the reason for it is.

What's the difference between these two examples please? They both output the same css but the first doesn't allow classes only elements. Why is the first example in use in some places?

#{h1, h2, h3, h4, h5}
{
  color: #000;
}

h1, h2, h3, h4, h5
{
  color: #000;
}

Answer 1

#{} is used for string interpolation: http://sass-lang.com/documentation/file.SASS_REFERENCE.html#interpolation_

There is one exception to this, though: when using #{} interpolation, quoted strings are unquoted. This makes it easier to use e.g. selector names in mixins. For example.

So this technique is used sometimes to allow using sass values in selectors. E.g.:

$gutter: 10;

.grid#{$gutter} {
    background: red;
}

Now to your question. I really don't see any reason why would anybody use string interpolation in this selector:

#{h1, h2, h3, h4, h5}
{
    color: #000;
}

My best guess is that sass variable will be added later to that selector, or the selector will be completely replaced with a variable.