The Toggle Comment item is available via the Reformat Text button on the collection toolbar and via the Format menu, except when the Recycle Bin tab is active. It combines the functionality of Comment Block and Uncomment Block.
On each line in the selection, Toggle Comment attempts to remove the comment characters exactly like Uncomment Block does. If it does not remove any comment characters from a particular line then it adds comment characters to that line just like Comment Block does.
If only one line was selected or if there was no selection then Block|Toggle Comment moves the text cursor to the next line. This allows you to quickly comment and uncomment multiple lines without having to select them simply by pressing the Ctrl+/ keyboard shortcut.
The advantage of Toggle Comment is that it allows you to use the same keyboard shortcut for commenting and uncommenting lines. But if you need to comment out a larger block in which some lines are already commented out that need to stay commented out, then you need to use Comment Block instead.