Which key code is used to enter a supplemental record after completing a base record?

The key code used to enter a supplemental record after completing a base record is "SU." This code specifically designates that the information being entered is intended to add to or enhance an existing base record, providing additional context or data relevant to that record. Supplemental records are important in tracking ongoing investigations or updates to cases, allowing for comprehensive data management within the system.

Using the correct key code is crucial, as it ensures that the information is categorized appropriately in the system. Other codes, such as those indicated in the other options, do not serve the specific purpose of creating supplemental records and are instead used for different functions like entering new records or making different types of updates. Understanding the specific role and application of the "SU" code is vital for effective data entry and record-keeping within the NCIC system.