are displayed in the FeatureManager design tree of the
assembly. You can hide or show all of these in-context icons by right-clicking
the top-level assembly icon 
and selecting Hide
Update Holders or Show Update
Holders.An external reference is created when one document is dependent on another document for its solution. If the referenced document changes, the dependent document changes also.
In an assembly, you can create an in-context feature on one component that references the geometry of another component. This in-context feature has an external reference to the other component. If you change the geometry on the referenced component, the associated in-context feature changes accordingly.
An in-context feature updates automatically if the update path is available. The update path is contained in the assembly where you create the reference. If the update path is not available (for example, if the assembly document is closed), updating occurs the next time you open the assembly that contains the update path.
Update paths for in-context features
are displayed in the FeatureManager design tree of the
assembly. You can hide or show all of these in-context icons by right-clicking
the top-level assembly icon and selecting Hide
Update Holders or Show Update
Holders.
You also have
the option to not
create external references when designing in the context of
an assembly.
In the FeatureManager design tree, any item with an external reference has a suffix that indicates the status of the reference:
The suffix -> means that the reference is in-context. It is solved and up-to-date.
The suffix ->? means that the reference is out-of-context. The feature is not solved or not up-to-date. To solve and update the feature, open the assembly that contains the update path.
The suffix ->* means that the reference is locked.
The suffix ->x means that the reference is broken.
See Locking and Breaking External References for more information.
To list the external references on a part or feature:
Right-click the part or the feature with
the external reference or right-click the update holder , and select List External
Refs.
The referenced components, features, and entities are listed, as well
as the status of each reference. See Rules for External
References for more information.