The circumflex is a coronary artery, supplying oxygen to the
heart muscle, at the rear/left side. It is an important vessel because it does supply quite a substantial amount of tissue. Severe disease refers to the fact it is coated inside over a considerable distance with plaque, restricting the flow of blood.
The Cardiologist may feel bypass is required. I do believe if the disease is over a long length of the vessel, it is very rare for stenting to even be mentioned.