The idea behind Top-Down design is to try to build in intelligence between the fit, form and function of parts that reside in an assembly. You try to capture this fit, form and function into the assembly first, and then pass the appropriate information down to the part level so that a change made at the assembly level or to one component in the assembly can drive updates to the rest of the critical parts.
In this method, many of the components are created in the assembly, instead of being assembled into the assembly.
The best way to capture fit, form and function for the assembly is to create a special kind of component called a Skeleton Model. A Skeleton model is similar to a regular part, but it is treated specially in the assembly. For example, a skeleton model is automatically excluded from the Bill of Material, where if you just created a regular part and used it like a skeleton, it would still be reported.
There are also restrictions that can be placed on regular parts that skeleton models are exempt from, or get special rights to deal with. For example, you can make it so you are not allowed to copy surfaces from a regular part to another regular part, but you can still pass surfaces from the skeleton model to a regular part. In defining such restrictions, you avoid creating parent-child relationships between individual part files, making the model more robust.
To demonstrate this principal, we will first create a new assembly file called Stacker. Be sure to use a Design sub-type for this assembly, just as we did for the last lesson. Also, once you have the assembly started, be sure to turn on the features in the model tree.