# External IDs
To support resources from external systems, you can reference the resources using externally-controllable IDs in the
externalId field in Cognite Data Fusion.
To learn more, see Define unique IDs with externalId.
# Asset Data Insight
To support changes to hierarchies, Asset Data Insight requires that you use the
externalId field when possible, for example, for time series.
If you don't use the
externalId field, different areas of Asset Data Insight may not work as expected.
For example, a time series without an
externalId can not be plotted on a chart.
externalId of a resource is changed in CDF, then any references to it will be broken.