STEP files usually have separate trees of both product and shape structure, with references back and forth between them. The product structure describes the bill-of-materials layout of an assembly, while the shape structure describes the geometric transforms and spatial location of the geometry for each assembly component. The viewer product and shape tree shows tries to show as much of this structure as it can.

Top-level STEP products are shown first, followed by any stand-alone shape objects. If a particular product is an assembly, such as the "VACCASE_ASM" product to the right, the tree entry contains a list of sub-component products. If the product has an associated shape, that is listed after any sub-components. The same shape or product can appear many times in the tree, if it is reused in the assembly.

The STEP shape objects may have relationships too, which contain geometry transforms and can be linked to the product structure. If you are interested in the underlying STEP instances in the STEP file, tooltips on the tree will give you the IDs of the important objects.

There are several ways of describing the shape relationships in a STEP assembly. Some systems use a context-dependent shape representation object to link the shapes, while others use a mapped-item object. Others attach B-Rep shapes indirectly through another empty shape. The STEP Desktop Viewer supports all of these, and will even try to make sense of things when a system does not follow any of the known styles.