What does a pen register record?

A pen register is a device used to record the numbers dialed from a specific telephone line. Its primary function is to capture the outgoing dialing information without intercepting the voice content of the calls. This means that while it logs the numbers that the target phone dials, it does not record any conversation or identify the participants in the communication.

The importance of understanding what a pen register records is that it helps in investigations where understanding the communication patterns of individuals is necessary. This tool is particularly useful for law enforcement to gather data regarding a suspect's contacts without needing to monitor the actual content of conversations.

This choice accurately reflects the pen register's capability, while the other options do not represent its function. Duration of a call and call participants relate to different types of monitoring methods that record more than just the basic dialing information. Incoming call locations pertain to a different facet of call data that is outside the scope of what a pen register captures.