These labels are a part of structures such as while loops and case structures. Unlike free labels placed inside the structure, a subdiagram label automatically resizes with the structure.
You can also color the label using the Coloring (aka paintbrush) tool. Select the Coloring tool using the Tools palette, which is shown using the View>>Tools Palette menu.
Since you can also color the structure itself, you can create interesting effects such as this:
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