Data Management Capabilities > Using Excel to Export and Import Data > About Importing Data from Excel > Import Logic When Importing from Excel
  
Import Logic When Importing from Excel
When importing data from Excel, circumstances may affect how your data is imported. The following steps described the logic used by the system.
1. Upon export, a hidden attribute is added to usage links in order to maintain usage link relationships when importing and exporting. First the system looks for an exact match to the usage link attribute.
2. If no matching usage link attribute is found, import identifies usage based on number and line number.
3. The next step is controlled by the following property: com.ptc.windchill.ixb.importer.bom.oldBOMSheetImportLogic
When the property is set to false, which is the default, when no matching usage link attribute is found a new usage link is created.
When the property is set to true, if no matching usage link is found, import identifies usage based on number.
In a rare case where multiple usage links are found matching the number, the import randomly selects one to update. The system tracks that this usage link has been updated such that the next occurrence of this same child part will update the other usage links for this same part.
If no usage links are found, a new usage link is created.  
The following scenarios also describe how import logic is used.
If a part does not have any child parts in the Windchill PDMLink system, but it does in the spreadsheet.
Upon import, the system creates a new usage link.
After you export a spreadsheet, a different user updates the assembly in the Windchill PDMLink system to create a new child part and remove an existing child part.
Upon import, the child part that was removed by the second user is added back into the Windchill PDMLink system because it existed in the spreadsheet.
The child part that was added by the second user but not defined in the spreadsheet remains in the Windchill PDMLink system. (Though it did not exist in the spreadsheet, the part is not removed.)
A child part is marked as removed in the spreadsheet. At the same time, the part is manually removed in the Windchill PDMLink system.
Upon import, the log identifies that no match was found for that part and that the action was ignored because there was no part to remove.