What does "acceptable surface defects" mean?

Making a membrane for MMDM is a complex process, and it's practically impossible to obtain membranes without any defect. These defects are the result of (dust) particles and are visible as small spots of uncoated membrane or as small particles stuck to the surface. Usually they are near the membrane edge. Up to two defects on a deformable mirror is considered to be OK, as they do not impair the mirror functionality. Please see Section 2.15.1 on page 42 of our Product Guide for more details.