In a shared assembly structure, the subassemblies and parts within the shared assemblies are also shared. This is because the components of a shared assembly belong to its unique "contents" information and not to any individual "instance" information. Therefore, changing instance information (such as position or part color) of a component of a shared subassembly affects all representations of the component within the structure.

Creo Elements/Direct Modeling provides a way for you to "split" a representation of a component to define a selective instance within a specific assembly context. This allows the component to be treated as though it were an independent instance, yet retain the contents information unique to the shared components. In particular, the assembly structure itself is not disturbed.

A chain of selective instances is necessary to split a low-level part or subassembly from a high-level shared assembly. You can define the selective instances individually, or the chain can be created automatically by Creo Elements/Direct Modeling.

When you create a shared assembly, shares are also created of the parts and subassemblies owned by the source assembly. However, assembly components belong to the contents information of the assembly, not to each individual instance of it; therefore the components of shared assemblies are not represented by multiple instances. For this reason, a change in instance information (such as position or part color) of a component of a shared assembly is reflected in all representations of the component in the other assembly shares.

For example, the figure shows a simple piston and cylinder model. Each cylinder, piston, and pair of piston rings comprise an assembly; the model consists of two of these assemblies shared. When one of the pistons is moved within the housing, both pistons move (right graphic). This is because the piston belongs to the contents of its owning assembly, which exists only once. Although the piston is displayed in two representations (denoting the two instances of the shared assembly), there is really only one instance of the piston itself.

A shared piston and cylinder assembly

Creo Elements/Direct Modeling provides a way to "split" representations of components of shared assemblies. By creating a selective instance (or by selectively unsharing), you define independent instance information within the context of one assembly instance without affecting the other assembly instances or the contents data. In this way, the components remain shared, but specific instance information can be set for the selected representation.

Note that a selective instance is specific to the context of the assembly instance in which it was created. In particular, selective instances are not automatically created for the other representations of the component. Therefore, if you change the position of the component within another instance of the shared assembly, the component will move in all of the assembly instances except in those in which a selective instance has been defined.

In the example of the pistons, by creating a selective instance of one of the pistons, you can then move it independently of the other piston; see figure.