When you solo a node, the Material Viewer automatically switches to flat view, displaying a flat plane swatch with only ambient lighting. The viewport also updates to indicate the soloing results.
The Material Viewer automatically switches back to a 3d geometry swatch when you remove soloing (for example, a sphere or a teapot, depending on the geometry that you selected).
When you solo a bump2d node, it remains as a 3d geometry swatch to allow you to better examine the bumps under the influence of lighting.
If you solo a material node, the Material Viewer renders your material as a 3d geometry swatch.
This feature allows you to examine all parts of your texture like you could in a compositing software, without any part of it being obscured by corners or backfaces. It also allows you to examine the texture on its own without being influenced by lighting effects such as shadows and reflection.
If you solo a node that is not part of the active shading network, the Material Viewer updates to reflect the soloed node in flat view and the viewport updates to reflect the soloing results. In other words, soloing takes precedence over selection.