What is the primary characteristic of the auto matrix?

The primary characteristic of the auto matrix is that it is pre-sized and does not require a retainer. This feature makes it convenient for dental procedures, as the matrix does not need adjustment to fit different tooth sizes, simplifying the process for dental assistants and ensuring a quicker setup during restorative treatments. The auto matrix is designed to wrap around the tooth and hold the material in place during the curing or hardening process without the need for additional retaining arms, which can streamline the workflow in a dental office.

In contrast, the other aspects listed relate to characteristics that are not applicable to the auto matrix. For example, auto matrices can be used for various types of teeth, not limited to just adult teeth, and they are specifically designed to fit securely around the tooth to create a proper form for the filling. Additionally, while installation does require some care to ensure proper seating, it is not considered special handling compared to more complex systems. Thus, the distinctive feature of being pre-sized and not requiring a retainer stands out as the defining trait of the auto matrix.