a great article by the well-known Thai massage and theory guy based in the US –> Bruce Bently recommended by my teacher!

Six reasons why a cupping mark is not a bruise
The list below is a brief summary of some of the critical differences between a cupping mark and a bruise. On many occasions cupping produces no marking, even when a robust volume of negative pressure (vacuum) is within the cup, because pathogens and other unwanted factors are not there to be drawn to the surface.

By definition, a bruise occurs “as a result of a blow that does not break the skin” (Lackie, 2010), or otherwise, “bleeding in soft tissue resulting from a direct blow with a blunt instrument” (Kent, 2007). The lifting suction effect of a cupping vessel on the skin stands in stark contrast to the
inward sinking dynamic of a blow to the surface. Furthermore, there is no trauma caused by the solid rim of a cup.

“A bruise changes colour, first to blue as a red pigment of hemoglobin loses its oxygen, and then to brown or yellow as the hemoglobin is broken down and reabsorbed”(Kent, 2007). This description of a bruise’s colour changes does not apply to the fading and resolution of a cupping
mark. The fading of a cupping mark is a progressive lessening of the original hue without any different colour transitions.

When we have a bruise, experience tells us that it is tender to touch (due to trauma). After cupping there is no such accompanying tenderness.

Note: Only following over zealous and aggressive cupping can a bruise-associated yellow stain be produced, together with tenderness within and beyond the periphery of a cupping mark. Such a sign represents poor practice.

Imagine a cup has been applied and produces a strong dark mark. After that has resolved and another cup is reapplied on the identical location, with the same suction level and for the same duration, the marking is typically only about half as ‘ferocious’ as the first time. By the third
session, chances are the response will only be a faint showing. Usually by the fourth
treatment no marking occurs. This is clearly a case of an internal pathogen or toxin being systematically resolved. This scenario would be the opposite if it were a
bruise. The capillary damage incurred by a trauma to produce a bruise would increase
with repeated assaults. The following is a
further illustration.

A bruise can be successfully cleared using cupping. Healing a large bruise for example, can be accelerated by applying a soft to moderate strength cup to the center of the bruise and sliding it outwards beyond the bruise’s perimeter. Complete the treatment by repeating the above along successive margins as if following the spokes of an imaginary wheel. A bruise is after all a form of blood stagnation and cupping is excellent at dispersing blood stagnation.