What is a significant limitation of a pen register in terms of capability?

A pen register is a device that captures data about communications, such as phone numbers dialed from a particular phone line, without recording the actual content of those conversations. This means that while it can track metadata related to calls—like the time a call was made, the numbers involved, and possibly the duration—it does not provide any insight into the substance of the communication itself. This limitation is significant because it restricts the ability to gather intelligence from the actual conversations that might be taking place, leaving only a record of the communication activity without contextual information.

The other options reflect aspects of pen registers that are either inaccurate or less fundamental to their primary functionality. For instance, while call duration might be a parameter recorded, it does not capture the critical limitation of not recording communication content. The time operation limitation and the physical presence requirement do not generally define the overall capability limitations of a pen register as effectively as the inability to capture communication content does. Therefore, the most significant limitation is indeed its lack of ability to record the actual communication taking place.