To create a Symbol, select a group, Artboard, or a selection of layers and choose Create in the toolbar, Layer › Create Symbol in the menu, or press ⌘Y on your keyboard. Give your Symbol a name and choose whether to send its Source version to a separate Symbols page or keep it on the page you’re currently looking at.
Symbol Sources and Instances
When you create a Symbol, we’ll create both a Source and an Instance of it. You can create as many instances of the same Symbol as you like, but you’ll only ever have one Source. If you make a change to a Source this will also show in any instances of that Symbol in your document.
If you choose Send Symbol to Symbols page, you’ll see a new page above the Layer List called Symbols. In here, you’ll find your Source.
If you didn’t choose to send the Symbol to the Symbols page when you created it, it’ll appear on the current page as its own Artboard.
How to turn a Symbol back into layers
To turn a Symbol back into regular layers, select the Symbol’s Source in the Layer List and choose Layer > Convert Symbol to Artboard. Any instances related to it will become groups and editing the Source will no longer change those instances.
How to replace a layer with a Symbol
When you’ve created a Symbol, you can insert in place of any existing layer in your design. To do this, Control-click on a layer, choose Replace With, then choose a Symbol. The Symbol you choose will resize to fit the layer’s size.