Tip: Avoid changing files that originate from other departments or tools.
Handling of input models often cause challenges in case they are incomplete or incorrect. For example, vehicle communication definitions often need to be synchronized for all network nodes and are modeled centrally, while each application team is expected to extend the model with a detailed application design. If the input model is not stored in other files than the extended parts, there is often a time-consuming and error-prone step of reapplying necessary patches and extensions each time the input model is updated.
It is thus important to store the input model in other files than the extension models, so that for example the vehicle communication model and the application design model are stored as different artifacts. This also simplifies updates of the input models since the files simply can be replaced with the new ones. If the input files are version controlled, any necessary patches can be easily distinguished when the version control system marks the file as changed and fed back to the team responsible for the input files. Alternatively, the input files can be marked as read-only to hinder unintentional changes. Depending on the strategy used, this could, however, potentially constitute a cumbersome strategy if the input files are updated often. In order to extend a model element or a package in another/new file without modifying its current file, use the context menu command 
Split to File....
Related Tool Features: File Locations
There are two features that help to identify in which file an AUTOSAR Element is stored. These features can be helpful to avoid making changes to files that are not supposed to be changed:
Use the Link with Editor feature of the DaVinci Project Explorer to reveal which files that contain an AUTOSAR Element selected in any comfort editor or the AUTOSAR Model Explorer. Refer to section DaVinci Project Explorer for more information about the feature.
View the Location property within the Status tab of the Property View to see the full file path of files that contain the selected AUTOSAR Element.
Related Tool Features: Read-only
Individual ARXML files or complete DaVinci Adaptive projects can be marked as read-only. For details refer to Element Locking.
Related Tool Features: Split to File
Many model elements can be split into multiple files. In any comfort editor, the AUTOSAR Model Explorer or the DaVinci Project Explorer, open the context menu of a splitable element and click Split to File... to split the element to another file.
For a practical instruction, refer to the Splitting AUTOSAR Elements Across Multiple Files cheat sheet.