You obviously upgraded from a previous version of UltraEdit.
Hide Lines and Show Lines are features which are mainly used for filtering large logs. For example administrators of a website may use these 2 buttons to filter the access log of the website, for example when just lines containing status code 404 (error code - file not found) should be displayed and all other should be hidden. If you don't need them, ignore those 2 buttons.
You prefer the option List Lines Containing String. In UltraEdit prior v18.00 using this option resulted in opening a resizeable, modeless dialog window with a list of all lines containing the searched string. Since UE v18.00 the lines containing the searched string are listed in a view with the title Find String List.
A view window has some advantages in comparison to a resizeable, modeless dialog window.
While both windows can float over a document window in any size, only a view window can be alternatively also docked on any side of the UltraEdit main window. When the Find String List window is docked for example at bottom, it is possible to configure this view to auto-hide itself. This has the advantage that the document window is not partly covered by the results of the find. Simply moving mouse pointer over the tab of the Find String List tab results in opening the view again, double clicking on one listed line results in setting the caret to beginning of this line in document window, and moving mouse pointer away results in hiding the results window again.
But even if you want the found lines always visible and therefore don't dock the Find String List window and don't use auto-hide feature, you have now the choice if the found lines should be displayed in a floating window over the document window like before or perhaps better docked at bottom of the UltraEdit main window so that never lines in document window are partly covered by the results of the find.
Another advantage of a view window in comparison to a resizeable, modeless dialog window is the fact that the view window can be reopened at any time after closing it still showing the lines of last find after reopening while the resizeable, modeless dialog window once closed could not be reopened. If you ever close the Find String List view window before having all lines evaluated, you can reopen it via menu item View - Views/Lists - Find String List.
A disadvantage of a view window in comparison to a resizeable, modeless dialog window is that it cannot have other elements like buttons. IDM compensated this disadvantage by giving the view a context menu with all the commands available before in the dialog window via buttons. So simply right click into the Find String List view window and you will see the command Copy to Clipboard which you want to use.