FBH means "Flat Bottom Hole", which is a standard reference reflector used in ultrasonic testing. When an actual defect is detected, the amplitude of its reflected echo is compared to echos of flat bottom holes of various diameters. This gives some indication of the size of the defect.
But the sound field from a UT probe spreads as it propagates, decreasing in intensity. Also, reflections from defects spread on their travel back to the probe. Therefore, as a defect moves farther from the UT probe, its echo amplitude decreases. In order to maintain some correlation between the defect echo amplitude and the size of a defect, a correction must be made for the distance between the probe and the defect. This is a DAC, or Distance Amplitude Correction. The DAC is a function of the probe and defect characteristics. Therefore, the FBH (Flat Bottom Hole) is used for reference, and a DAC is developed for the probe characteristics being used.
I hope this helps,