How to enable Cloud Compare's Fine Registration (ICP) command

For the longest time, I tried to align two point clouds with CloudCompare's Fine Align (ICP) command. But unfortunately, the command would not enable for me no matter what I did. I selected one cloud, two cloud (or so I thought). Neither reading the documentation nor watching the YouTube videos helped. The screenshot below showed the deactivated Fine Align command in the tool bar.

It turned out that proper selection is key. I was using the SHIFT key to select one cloud folder node and another cloud folder node, which selected folders, and clouds among other things.

1. The proper way was to press down the CTRL key while clicking on only cloud nodes in the tree view pane, as shown in the screenshot below. Once two cloud nodes have been selected, the File Align icon in the toolbar will become enabled.
   
   The File Align icon is enabled.
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2. Click the Fine Align icon.
   
   The Clouds registration dialog box appears.