Harper Lee identifies many different varieties of the social realm of 1930s Maycomb in her novel To Kill a Mockingbird. There are obviously distinct differences between the three primary groups discussed: middle class whites, lower class whites, and Negroes. However, the middle class society of Maycomb also show a wide variety of behavior. Most of the whte characters in the novel seem to show the typical racial prejudice against blacks, but many of them also display hatred and/or disillusionment toward other groups as well. Jews, Yankees and even Alabamans from the northern part of the state are criticized by various characters. Harper Lee quietly shows how women play a second-class role to their male counterparts. Children are also criticized (Mr. Avery even claims they are a type of jinx.) Poor Southerners are considered beneath the many of the characters, particulary Aunt Alexandra. The many different groups are expected to display specific behaviors, and for the most part, most of Maycomb doesn't exactly agree with Atticus' belief that there's only one type of folks: "Folks."
Propriety means conforming to a social standard. When we refer to society, its almost implicit in its meaning that there are expected standards and norms in each society.
The citizens of Maycomb serve several purposes and uses in TKAM. Lee uses the town to demonstrate a caste system (easily seen in almost any society) that affects the relationships of the people within the different classes. Further, she uses the society to show what's acceptable and what wasn't. For example, she used Dolphus Raymond to show the people's disdain for mixed relationships. She used the ladies Missionary Society to illustrate the judgment and hypocracy that marked the post-slavery South.
The norms demonstrated here were to stay within your class' expectations of behavior, and not to mix races under any circumstances. These norms portrayed the immorality of that society at that time.