A surgical marker is just that — a marker, usually using gentian violet ink as its coloring agent. Surgical markers are generally packed sterile (sometimes with a ruler as well), but because sterile surgical markers cost about $2.00 each, many studios reuse them between clients (or use gentian violet on a toothpick, another common marking method). Non-sterile surgical markers are also available.
Because the Hepatitis B virus is present in sweat, tears, saliva, and genital discharges when the person is in the viremic phase, it is possible to transfer Hepatitis B from unbroken skin to a surgical marker. On the next client, this can be then transferred from the marker to the skin, and then when the skin is penetrated, into the bloodstream. Other microbes can be passed similarly.
Admittedly, the chances of this happening are slim, but so are the chances of being hit by lightning (although it must be mentioned that about 200 people in the USA are killed by lightning every year). It is generally considered that anything used to mark the skin in a tattoo or piercing procedure must be single use.