Your interviewer will likely ask you situation- and knowledge-based questions to understand how well you are able to explain concepts to others. Be prepared for questions like this one by practicing a few possibilities ahead of time. In this case, flatness refers to a type of Geometric Dimensioning and Tolerancing (GD&T) callout. This is a specific parameter that refers to the distance between two parallel planes that define a surface profile. It's a measurement of how flat a surface is relative to its own shape, and it doesn't require a reference plane or datum (unlike the GD&T parallelism callout). Therefore, the part must be secured such that the measuring tool is parallel to the nominal surface measurement. Measuring profiles that are not nominally flat can usually be done with a computer-controlled probe on a machine called a CMM, which compares the probe measurements to a 3D Model to measure the variation of the surface profile.