Nested Symbols can help streamline complex projects by giving you a more granular control over your designs. For example, a nested Symbol can take the place of a design element, such as a logo, within a larger Symbol like a banner. This way, any edits you make to your logo will automatically reflect on your banner’s Symbol Source.
How to create a nested Symbol
You can create nested Symbols by inserting a Symbol instance inside a different Symbol Source.
To do this, go to your Symbols page and insert an instance inside your Symbol. Now, when you edit the Source of that instance, it’ll update everywhere, including inside your Symbol Source.
You can also turn two or more Symbol Sources into nested Symbols by selecting them on your Canvas and choosing Create Symbol 
in the toolbar.
Note: There’s no limit to how many Symbols you can nest within a Symbol Source. However, you can’t use a Symbol instance inside its own Source.
How to use nested Symbol overrides
For even greater customization, you can create other versions of your nested Symbol to use as overrides. For example, you can turn several icons into nested Symbols. Then, whenever you want to swap one icon with another, you can select it from the Overrides section of the Inspector. Here’s an example:
Example of using overrides to change different nested Symbols within a Symbol instance in the Mac app.
To quickly access the Symbol Source for one of the nested overrides, click the arrow icon 
next to the nested Symbol override’s name in the Inspector.
Example of using editing a nested Symbols within a Symbol instance in the Mac app.
Overrides for nested Symbols reflect the structure of the nesting. The Inspector groups overrides by type — so you’ll see all your Symbol’s color, text, image and Style overrides together, as well as nested Symbols and Hotspots. You can override these by selecting the entire Symbol instance, or selecting the individual nested Symbols or layers within the Symbol to override specific elements.