== A. Instances ==
Once you have a symbol, you can then use them in other symbols, or on the main stage. An occurrence of a symbol outside of the library is called an 'instance'. Virtually all of the work done in Flash is done with symbol instances. To create an instance of a symbol, just drag and drop from the library to the stage. This process can be repeated many times, thus promoting a simpler work load, as well as a smaller file size.
== B. Combining symbols ==
While an instance cannot be placed inside its own symbol, it can be placed inside other symbols. This is a good practice when building complicated structures, such as a web page. Symbol instances, such as buttons and content can be dropped into a page symbol, and then that super-symbol can be dropped onto the main stage in one step, rather than the several steps it would take with the individual symbols themselves. In the example, I have combined the cat, tree, cloud, house, and BG symbols into the cat scene symbol, and then dropped the cat scene symbol onto the stage.
== C. Global changes to symbols ==
If you want to change all the instances of a symbol, say to make all the green buttons blue, you can edit the original symbol and all instances of that symbol will show the change. To edit a symbol, just double-click on its name in the library, and a screen will just show that symbol. In this example, I painted the cloud purple, and you can see that all the clouds are now purple in cat scene.
== D. Changing individual instances ==
There are two ways to change individual instances; you can either apply filters to the instance, or make it into a new symbol.
Select the instance you want to change, and go to the filters drop-down in the properties panel. Select the attribute you want to apply, and then modify it. These apply to the whole instance, so you may need to go to a lower level (to one of its component symbols) to make the desired changes.
b. New symbol
You can also change any instance to a new symbol. Use any of the aforementioned methods for symbol generation and make your desired changes.
== E. Naming instances for ActionScript ==
If you are going to be using ActionScript in your project, you will need to name the instances you will be targeting. Simply select the instance, go to the properties panel, and type in a new instance name. It is advisable to use plain English names that make sense. In the example, I named the instance 'cat_mc', because it is a cat. The '_mc' means that the instance is a movie clip symbol. There are standard extensions for each symbol type, such as _txt and _btn. While not strictly necessary, it is good form, and helps Flash give you the appropriate code hints.