It's a 'Layout' icon.
Creating a Layout-based Assembly Design
In layout-based assembly design, you can switch back and forth between top-down and bottom-up design methods. You can create, edit, and delete parts and blocks at any point in the design cycle without any history-based restrictions. This is particularly useful during the conceptual design process, when you frequently experiment with and make changes to the assembly structure and components.
To create a layout:
- Open a new assembly.
- In the Begin Assembly PropertyManager, click Create Layout. A 3D sketch opens, and Layout appears in the FeatureManager design tree.
- Create sketch entities and group related entities into blocks.
- Close the blocks and the sketch. You can create a layout in an existing assembly by clicking Create Layout (Layout Tools toolbar).
Oh, okay. Is there any particular difference between it being white (like it is in my pic) and yellow (like it is on the SW help page)?
Not that I'm aware of but then I'm on SW2015, before color change.