The civil service exam was the main route to upward mobility in China. Anyone who "passed" it who be a candidate for official posts in China. There were several levels, and the higher the level of the pass, the higher the level of eligibility. A person who had passed an exam and held no post was ranked higher socially than a person who had not passed an exam, and held a good post in "private" life, because the first person was eligible to be an official, and the second person wasn't. In parts of Europe, for instance, an academic is held in higher esteem than a businessman, even if the latter is richer.
One such exam passer at the highest level was this man, who happens to be my father's maternal uncle: http://en.wikipedia.org/wiki/Liang_Shiyi.
A person who didn't pass a civil service exam would revert to his original status, peasant, laborer, merchant, or member of the gentry. For people at the bottom, passing an exam was basically an "all or nothing" proposition, because there were few other avenues of advancement in China.