What is used when additional space is needed to include all related incidents?

The correct choice is the DHS form 3155 continuation, which is specifically designed to provide additional space for documenting related incidents when the initial report format does not have enough space to capture all necessary details. This ensures that all relevant information can be accurately recorded and that incidents are documented thoroughly for further analysis and action.

Using the continuation form allows for comprehensive reporting without the need to omit critical details, which is vital in security operations where every piece of information can be significant for assessments and future responses.

The other forms listed serve different purposes and do not provide the same functionality for expanding incident reports. For instance, DHS form 1103 is focused on other specific reporting needs, while DHS form 139 is tailored for different types of entries related to security incidents, and DHS form 3155 serves as a standard reporting form without the added capacity that the continuation form offers.