Block references that contain other blocks are known as nested blocks. Using blocks within blocks can simplify the organization of a complex block definition.
With nested blocks, you can build a single block out of several components. For example, you can insert as a block a drawing of a mechanical assembly that contains a housing, a bracket, and fasteners in which each fastener is a block composed of a bolt, washer, and nut. The only restriction on nested blocks is that you cannot insert blocks that reference themselves.
You can apply geometric constraints and constraint parameters to nested objects in blocks. AutoCAD detects the nested entity or valid constraint point for the nested entity regardless of the nesting level of the object.
When a block definition is redefined, AutoCAD will re-evaluate the constraints between geometry in the drawing and the nested geometry in the block references. The drawing will then be updated appropriately. If a constraint cannot be resolved as a result of the change to the block definition, then the constraint is removed and an unresolved constraints message is displayed at the command line.