The document discusses cylindrical coordinate systems. It defines cylindrical coordinates as having three variables: ρ, φ, and z. A point is located by specifying values for each of these three variables. It also defines the unit vectors aρ, aφ, and az that make up the cylindrical coordinate system. Finally, it mentions that a differential element dl in cylindrical coordinates is represented as dρaρ + ρdφaφ + dzaz.