Model-to-instance association refers to the link that is established between a part in Model mode and a drawing instance in a layout. When you create a layout, modify the part in Model mode, and then return to Layout mode, the association between the part file and the instances will be recognized. Each of the layout's instances is automatically updated to reflect the new state of the model.

For instance, a part is created in Model mode that consists of lines and arcs. You then enter Layout mode and create a layout showing different views, or drawing instances, of the part. Perhaps, then, you decide to make modification to the geometry in Model mode. When you return to Layout mode, the geometry is automatically updated in all of the layout instances. However, if you make changes in Layout mode, you cannot see those changes when you return to Model mode. That's because any new entity that you create in Layout mode immediately becomes layout-specific, or specific to that layout. The new entity has no reverse ties to the original part in Model mode.