Over the past century, the word 'ground' has acquired so many different meanings and countless more incorrect usages that I'm totally confused as to what these devices do!
So using USA terms is it a?
A path to the dirt/soil. (GEC) Grounding Electrode Conductor - the wire to Mother Earth that attempts to keep everything at near the same potential during a thunderstorm or a power company high-voltage failure.
The meaning of ground hasn't really changed,except for the recent discovery of some companies that use ground for noise reduction. This method has been used for years by recording engineers and ham radio operators. Today you will find devices that use a "sink" in the operation which is ferrite material or bars of steel or cast iron. The EMI or current noise is produced in audio equipment and has a profound effect on music reproduction. The sink is so to speak a emi collector as noise current collects on the inside skin of the audio enclosure and if given a pathway will follow the path of least resistance to the sink. Of course no sink is really needed as a pathway to ground is actually the biggest sink imaginable, given the the earth has always been ground. What is the deciding factor is the size or circumference of the pathway used and plain copper wire has always been the best choice as a conductor. Since this noise current carries no audio signal and anything else used as a conductor is of no value. Creating a sufficient size pathway is all that is needed,and one could if wanted use up 1 gauge wire,but it is not really feasible,because of size and inflexibility. The wire size that is probably the maximum size for noise current is 4 gauge and two legs are best. Using 12 or 14 gauge is at best minimal and really a waste of time. Real noise current reduction has a far greater effect than any lower distortion figures in todays high end and in fact some equipment from as far back as the 1950's with noise current reduction can sound extremly well.
A path to the main circuit breaker box (EGC) Equipment Grounding Conductor - Safety Ground/Protective Earth - the path that trips a circuit breaker during fault conditions.
A audio circuit common.
A part of the shielding system.
A common reference point.
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Some thirty years ago, Henry W. Ott wrote an excellent paper on 'ground'.
"Ground- A Path For Current Flow"
http://www.hottconsultants.com/pdf_files/ground.pdf
The Henry W. Ott web-site.
http://www.hottconsultants.com/index.html
