In Starfleet, a "Crewman" can refer to either a person who holds the actual rank or simply any person who is assigned to an organization or starship. The grade of Crewman may further be divided into several "classes", such as "Crewman, First Class", "Crewman, Second Class", and "Crewman, Third Class". These subdivisions all use the same rank insignia.
In many cases, crewmen are refered to by their job description rather than their rank. For example, a crewman working in engineering may be referred to as "technician" or "systems analyst", while a crewman working in sickbay would be referred to as "nurse" or "medical assistant".