This simple dialog displays the list of available text variables for insertion in your topic. The list includes both Help+Manual's predefined variables and any variables you have defined for your project.
When you insert variables with this dialog they are highlighted in green in the editor, which makes them easier to identify. This is only a convenience, however. You can also enter variables by typing them directly in the editor. When you publish your output Help+Manual will still identify them.
Selecting More... displays the Text Variables section in your Project Configuration so that you can edit existing variables or add new ones.