What is the purpose of the second I.D. in an Inmarsat-B or F77 SES?

The purpose of the second I.D. in an Inmarsat-B or F77 SES is to provide an additional number that may be dedicated to computers, fax, and other types of data communications. In communication systems, especially those designed for maritime and remote operations, having a dedicated number allows for separate data services to operate simultaneously with voice communications. This setup enhances operational efficiency, facilitating both voice and data transmission without interrupting each other.

Using a second I.D. specifically for data applications ensures that devices such as computers and fax machines can send and receive information independently while still providing a primary channel for voice communications. This is particularly useful in maritime environments where timely access to data and communication can be critical for operations, navigation, and safety.

In the context of the other options, while some may touch on aspects of communication management, they do not accurately capture the specific function assigned to the second I.D. in the SES systems, which is primarily focused on facilitating dedicated data communications.