Symbols are a new feature in 1.5.x versions of Scribus. Exactly what they are intended for is a bit unclear, but this wiki page will hopefully spur someone to add or supply some further information.
The name is a bit unfortunate, since in DTP we might most often associate it with symbol glyphs in a font. Experimentation with the Symbol feature shows it to have an association with Patterns, though there may be some other uses. What I want to do in this article is go over some things I have learned about usage.
Sources of Symbols
It seems that almost any Scribus object can become a Symbol. Text frames, image frames, render frames, shapes, polygons, arcs, spirals, lines, Bezier lines, freehand lines, calligraphic lines, even PDF form checkboxes (and presumably other form elements) can all become Symbols. At least with a cursory trial, tables cannot become Symbols. An important thing to note about the conversion to a Symbol is that this cannot be reversed, in other words, you can't convert a Symbol into anything else. So if you want to convert an image frame into a Symbol, yet keep the original as an image frame, make a copy.
From the menu, choose Item > Convert > Symbol, or you can right-click the object for the context menu for the same result.
Editing a Symbol
A Symbol is a special object, or perhaps it's better to consider it not an object at all, in the Scribus sense. Your original item which you converted into a Symbol has some limited editing capabilities of that particular instance of the Symbol. These include the elements in the X,Y,Z tab of properties, and you may also create a drop shadow.
Additional editing is available from the Symbol dialog (from menu, Windows > Symbol). This will show all of your current Symbols from which you can for one thing, drag and drop to a page for another instance of your Symbol, or right-click and choose Edit item (or Remove item if you no longer want this Symbol). Double-clicking the item also brings up the editing window. If you remove the Symbol from the Symbol dialog, all instances of it are gone. One more option you have in the dialog is to right-click the white space outside the symbols to choose to show only the symbols and not their names.
Although it might seem convenient to bring a number of your symbols into this window to edit serially, you can only have one at a time. When it first opens, you might think you are ready to edit, since the symbol is outlined in red, but you need to click it before editing is possible. Once you do this, look to Properties to see all of your editing capabilities, since now you should see a number of tabs which will be selectable. Keep in mind that you will be editing all the instances of your symbol in this window.
What you can edit will be similar to what you could have edited in the original source. For the text frame, you can edit the font, linespacing, color, and the various items in the Text tab. You can even change the text. For images you may edit what is in the Image tab, and also load a new image with the context menu. Careful with Render frames – an initial experiment caused Scribus to crash when I tried to edit the content. In the Shape tab, you can select Text Flows, but for Symbols this doesn't seem to have any effect on underlying text frames. Given these initial comments, experimentation will show you what you can do with other kinds of content from other sources.