The ‘Tree/List Viewer’ is a general purpose tool for examining data trees of geometry.
In this case, you choose a path (data tree branch) using the ‘path idx’ slider and see the number of grid cells in that section (view direction) of a radial graph. Some sections have none. The ‘list idx’ slider lets you choose one of them.
Very handy tool for many (most!) GH projects.
P.S. The ‘Tree/List Viewer’ can be removed, it contributes nothing to the final output. It’s a coder tool.