Acquired similarity refers to traits that have been acquired or added onto a thing or being to make them appear more alike and appear to be in the same category.
An example of this is the use of uniforms by the military, many occupations, and some private schools to make the person's position, function, or affiliation in the community identifiable without singling anyone out as "special." This is opposite to the concept of acquired distinctiveness (hyperlink?) which is the addition of something (i.e. a skill or type of dress) that causes an individual or thing to singularize its presentation.