[KevinH]

Form_Files/ hold the user interface layout design for buttons, toolbars, menus and things for the various Sigil widgets. And main.ui is for the MainWindow (read that Sigil main window). So that AddExisting is for a menu or toolbar icon used for Sigil's.

The file and line number info can usually provide a bit of context if needed:
In this case: https://github.com/Sigil-Ebook/Sigil..._Files/main.ui near line 440.

Here is a snippet:

<widget class="QToolBar" name="toolBarAddExisting">
   <property name="windowTitle">
    <string>Add Existing</string>
   </property>
   <attribute name="toolBarArea">
    <enum>TopToolBarArea</enum>
   </attribute>
   <attribute name="toolBarBreak">
    <bool>false</bool>
   </attribute>
   <addaction name="actionAddExistingFile"/>
  </widget>

So yes, this string is for the ToolBar function for "Add Existing" in Sigil main window toolbar.