Structural encoding is the process by which humans encode and memorize and recognize faces. This is the mind's method of observing, memorizing and recalling other people and associating those people with their names. The mind is capable of recognizing very small differentiations between facial features and the ability to associate a name with a face.
Interestingly, this faculty works best for people when they are looking at features within their own racial/ethnic group. This is the basis for the commonplace statement "That all (fill in the blank ethnic group) all look alike." This is a problem with eyewitness identification a member of a different ethic group even though when looking at photographs of multiple members of an ethnic group it is obvious that there is a great deal of variety of features within any group of individuals.
Some individuals who have prosopagnosia or 'face blindness' have difficulty or the inability to recognize faces and structural encoding discrepancies may be involved in this.