Top-down design is also referred to as "in-context
design" in the SolidWorks Help.You can create assemblies using bottom-up design, top-down design, or a combination of both methods.
Bottom-up design is the traditional method. You first design and model parts, then insert them into an assembly and use mates to position the parts. To change the parts, you must edit them individually. These changes are then seen in the assembly.
Bottom-up design is the preferred technique for previously constructed, off-the-shelf parts, or standard components like hardware, pulleys, motors, etc. These parts do not change their shape and size based on your design unless you choose a different component.
Top-down design is also referred to as "in-context
design" in the SolidWorks Help.
In Top-down design, parts' shapes, sizes, and locations can be designed in the assembly. For example:
You can model a motor bracket so it is always the correct size to hold a motor, even if you move the motor. SolidWorks automatically resizes the motor bracket. This capability is particularly helpful for parts like brackets, fixtures, and housings, whose purpose is largely to hold other parts in their correct positions. You can also use top-down design on certain features (such as locating pins) of otherwise bottom-up parts.
The design of photocopier can be laid out in a layout sketch, whose elements represent the pulleys, drums, belts, and other components of the photocopier. You create the 3D components based on this sketch. As you move or resize elements in the sketch, SolidWorks automatically moves or resizes the 3D components in the assembly. The speed and flexibility of the sketch allows you to try several versions of the design before building any 3D geometry, and to make many types of changes in one central location.
The advantage of top-down design is that much less rework is needed when design changes occur. The parts know how to update themselves based on the way you created them.
You can use top-down design techniques on certain features of a part, complete parts, or entire assemblies. In practice, designers typically use top-down techniques to lay out their assemblies and to capture key aspects of custom parts specific to their assemblies.