________ are more reliable as mirror planes than faces.

Datum planes are indeed more reliable as mirror planes than faces due to their defined positioning and orientation within the modeling environment. They serve as reference elements that ensure consistent geometric relationships and can be used to establish a clear and repeatable system for mirroring operations.

In CAD modeling, datum planes can be created precisely and can exist independently of other geometric entities, which enhances their reliability during operations such as mirroring. They are integral to ensuring that any mirrored features will align accurately with the desired geometry, maintaining spatial integrity within the model.

In contrast, faces can be subject to variations based on the underlying geometry and can lead to unpredictable results if the geometry is altered or if the face is not planar. This variability makes them less suitable for operations that require a high degree of precision, such as mirroring complex features.

Thus, the reliability of datum planes as mirror planes is rooted in their ability to provide a stable and consistent reference, which is essential in rigorous engineering and design processes.