Sources of Block Definitions

1. Blocks defined in the current drawing. 
   
2. Other drawing files inserted as blocks into the current drawing. 
   
3. Blocks defined in other drawing files, called block library drawings, which can be inserted into the current drawing. 
   
4. Blocks created by online vendors and services. These blocks are often associated with specific parts or products. 
   

Insert Blocks

When you insert a block reference, you specify its location, scale, and rotation. Subsequent references to that block definition can be inserted in different locations, scales, and rotation angles. When a block is inserted into a drawing, it is scaled automatically according to the ration of units defined in the drawing to the units defined in the block. For example, if the unit of measurement is meters in the destination drawing and centimeters in the block, the block is inserted at 1/100 scale. 

Although a block reference is always inserted on the current layer, the block reference preserves the properties in the block definition. This means, for example, that blocks can have multiple colors even when they are inserted on a layer set to red or some other color. 

1. Ribbon Gallery
   
On the Home tab, Blocks panel, you can click Insert to display a gallery of block definitions in the current drawing. Use the gallery when you have a small number of blocks to insert quickly by clicking and placing them in your drawing. 

2. Blocks Palette
   
The Blocks palette is designed for fast and anytime access when you use a moderate number of blocks in a drawing. In the Blocks palette, sign in to your Autodesk Account, and specify a cloud storage location to sync and access your blocks anytime, anywhere. This palette displays blocks organized into four tabs: Current Drawing, Recent, Favorites, and Libraries. You can access the Blocks palette with the BLOCKSPALETTE command. 

3. Tool Palettes
   
The Tool Palettes window is designed for when you use a large variety of blocks. Many tabs are available and you can create custom tabs for sets of related blocks and specialized tools. You can insert blocks from tool palettes by dragging the block tool into the drawing or by clicking the block tool and then specifying an insertion point. You can access the Tool Palettes window with the TOOLPALETTES command. 

4. DesignCenter
   
The DesignCenter window is designed for browsing and selecting a variety of definitions from existing drawings and drawing libraries. These definitions include blocks, layers, linetypes, and other content. DesignCenter offers a quick, visual way to drag and drop blocks within the current drawing or from another drawing. Double-click the block names to specify the precise location, rotation, and scale of the blocks. You can access the DesignCenter window with the ADCENTER command. 

5. Drawings in Folders
   
Drawing files can be dragged and dropped into the current drawing from a file folder as blocks. 

You can remove block references in a drawing by erasing them, however, you remove unused block definitions from the drawing by purging them with the PURGE command.