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A symbol or part is placed into a drawing as a group; it has a unique name and is selected and manipulated as a single object. Because they are placed as groups, each instance of a part or symbol creates additional drawing data, and inserting numerous library items can increase your drawing size significantly. If you need to use many instances of a particular symbol or part, you can reduce your file size by making it into a block. A block is stored internally in a drawing, and references to it are inserted into the drawing rather than the objects themselves. See Blocks.
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You can modify these symbols (though it is preferable to modify copies and leave the originals intact), and also create and save your own symbols.
Symbol manipulation is done via the Library Palette.