Reading about the French Revolution. the Third Estate comes up a whole lot. They're described as the "everybody else," or as between 95 and 98 percent of the population, or as just the representatives of the common people. But were they really just average Frenchmen?
The Third Estate representatives who went to the Estates General in Versailles, formed the National Assembly, and approved the Declaration of the Rights of Man and Citizen were what the French referred to as the bourgeoisie. They weren't peasants, farmers, or poor people; they were what today would be described as upper-middle class professionals.
The bourgeoisie were educated dudes who'd been successful in their careers. A lot of the representatives were lawyers, but they also were businessmen and bankers. White-collar type workers who lived comfortably and didn't have dirty proletariat type jobs. They also mainly lived in cities as a growing urban professional class.
Some bourgeoisie were very wealthy, but they didn't have the privileges of the nobility. The bourgeoisie had to pay high taxes and had absolutely no say in the government of France. They were entirely left out of the decision making in their own country despite being a huge part of what kept it going. Furthermore, they were a product of the Enlightenment so as their king squandered France's resources they'd learned about alternatives to corrupt monarchies. They'd also read the news about Americans who were trying out some of those newfangled ideas about democracy.
The fact that it was primarily the bourgeoisie who voted for the Declaration of the Rights of Man and Citizen explains why they chose to specifically define it as only citizens who could reap the benefits of these rights. They were looking out for their own and unwilling to extend their ideals to lower classes of people who didn't own property and hadn't been to school. The Declaration of the Rights of Man and Citizen could easily be called the Declaration of the Bourgeoisie, as it's their rights it protects. They're the ones fearful of the government overstepping the law hurting those with the most to lose.