Each new element you add to your Customily templates has a Unique ID. This is the way we can identify elements.
Think of layers in Photoshop, where each layer has a name so you can identify it. Well in Customily we identify these elements with numbers, called IDs.

Each element gets assigned a Unique ID automatically starting in 1. This means the first element you create on your template (no matter if it's a text element, an image placeholder, or what) will have Unique ID 1, the second element Unique ID 2, and so on.

To know what's the unique ID of a certain element all you need to do is select it on the workspace. On the right, where all the information to format that element appears, you can see the element's Unique ID on the layers panel.