When you use the Archive Files button or context menu commands, the Add to Archive dialog box is displayed.
The top of the dialog is always the same and is where you select the archive type, path and name.
The Archive Type drop-down lets you choose from any of the archive formats that Opus is able to write to. (Some formats are read-only and won't be shown here.) Opus supports multiple archive formats, with Zip built-in and formats like 7z and RAR handled by a plugin. Third-party plugins can also add to this list.
The two text fields allow you to specify the location and name of the archive, respectively. Their drop-downs display your most recently used locations and archive names. (If you select an archive name from the history, its extension will be adjusted automatically if it doesn't match the type of archive you are creating now.)
As well as adding to a new archive you can also select an existing archive to add more files to it. The two Browse buttons allow you to locate existing folders and archives. Use the browse button next to the path to keep the name and only change the location. Use the browse button next to the name to select an existing archive; it will initially show you archives below the current path but you can also use it to select archives from other locations.
If your clipboard contains the full path to an archive you would like to use, you can paste it into the name field to automatically fill in both text fields at once.
To copy the full, combined path from both text fields, push Ctrl+Shift+C.
The bottom of the dialog displays options specific to the selected archive type. These options will change as you change what's selected in the Archive Type drop-down. Some archive formats don't have any options at all while other formats allow you choose things like compression modes and encryption options.
More:
Zip Options
7z Options
RAR Options
TAR BZip2 Options
TAR GZip Options