The Topic Selection dialog is used in a number of places in Help Builder whenever you get the option to select a topic. For example the on the Related Topics tab, to add a related topic you have to pick a topic. Usually you get an autocomplete list along with a button.

The dialog itself is like a mini version of the main Help Builder window.

This window lets you browse through the Help Builder topics in several views. The default view uses the same tree view list as the main window. Double click on the topic you want to select to select and exit this dialog.

If your search is a little more complex or you're not sure, you can click Preview to pop open this window and see the full topic displayed.

Notice that here I decided to run a search, which pops up a dialog to allow searching by keyword. The dialog displays a set of matches which when clicked highlight the topic in the treeview.