Here is a simple explanation using diagrams on how symbols work in Adobe Flash.
When you drag a symbol from your library on stage, it becomes an instance.
Making changes to the symbol will affect all instances on your stage that is connected to that symbol.
Adding an effect to an instance will not affect the other instance or symbol.
Symbols in Use
Using instances of a tree symbol on stage. This enables you to create a forest using only one tree.
Altering Symbols – In use
Changing the symbol affects all instances on stage that is connected to that symbol.