No one person or group sets the limits on the socioeconomic classes of the US, nor is there a set standard of limitations on any particular class. Change and limitations of classes or determined by the will of the American people.
The nature of socioeconomic limitations is determined by a combination of education, income and occupation. As such, limitations are determined by what the American population deems acceptable, and those limitations vary from person to person depending on area, family, ambition and a number of other factors. Opportunities for advancement and change within the system exist regularly and are often determined by more than situational factors. The belief is that anyone can move up the socioeconomic ladder in this country, but not all have as many advantages as others.