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.

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Making changes to the symbol will affect all instances on your stage that is connected to that symbol.

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Adding an effect to an instance will not affect the other instance or symbol.

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Symbols in Use

Using instances of a tree symbol on stage. This enables you to create a forest using only one tree.

 

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Altering Symbols – In use

Changing the symbol affects all instances on stage that is connected to that symbol.