If you have modified a project drawing that contains Plant objects when the project is not available, the project must be updated. These changes are known as offline edits.
When you open a drawing that has been modified outside of the project, the change is detected and merged automatically. However, there are additional update requirements when the drawing contains connections to other drawings. This includes Off-page connectors in P&ID drawings, and Piping connections made to external references (Xref) in 3D models:
To avoid breaking connections to other drawings, updates must be made in the same project session and before creating new connections.
Connection information from a drawing update is retained in temporary (placeholder connector) objects in the updated project. This information is used to recognize when another project drawing that contains the same line is updated. This information is lost when you close the project.