In 2012, candidates needed either one percent or 500 signatures (whichever was lower) from Democrats in each of the state's 50-plus congressional districts. They had two months to do so -- which, particularly if you're hoping to keep expenses under $5,000, is a time crunch. Other states work similarly; some don't.
A contemporary definition of rank-and-file is prevalent in labor union vernacular when referring to general membership and not union leaders. Rank-and-file generally means the employee doesn't occupy a high-level position within the organization.
In the Armed Forces, rank and precedence are generally determined by pay grade, however within the Army, specialist ranks immediately below corporal. So the following covers the paygrade and ranks, with images of rank insignia, starting with the low-level (or Entry Level) paygrades.