An electrical current traveling through a wire will generate a magnetic field. The orientation of that magnetic field is directly related to the flow of current in the wire. By tightly twisting two wires with opposite current flow you effectively cancel out any electromagnetic radiation from the wire pair. Structural return loss is a measure of the uniformity of the cable impedance. In general it is a measure of the cable quality.
Impedance is the resistance to the flow of alternating current.
The value will change with cable temperature. Most cables are certified at about 70 degrees Farenheiht. The attenuation to crosstalk ratio indicates difference in signal level between the received signal on a pair and the noise or NEXT on the same pair. This measurement is optional and can be created from the NEXT and Attenuation measurements. Noise is the measure, in dB, of the electrical interference in the vicinity of a cable.