I think you got a slight misunderstanding what the first one means.
.element .symbol {}
Means that those CSS settings are applied to any HTML element with the class .symbol
that is inside an element with the class .element
.
<div class="element">
<div class="symbol" />
</div>
In this example your first CSS entry would affect the <div>
tag in the middle.
Your second example means that the first class requires two classes to be affected. Other than that it's equal to the first one.
<div class="element large">
<div class="symbol" />
</div>
So if the HTML looks like this, the CSS values will be applied to the inner <div>
tag as well.
If you want to set CSS tags that apply for multiple classes separately then you need to split them up using a comma. So it looks like this:
.element, .symbol {}
Edit: By request the link to the documentation of the CSS selectors.