The orange line on the diagram shows, in conceptual terms, how insurance costs change as you move up the tower. An insurance coverage tower is made up of several layers of underwriters. The underwriter at the bottom of the tower (known as the primary) is first in line for any claims that might be made. Underwriters on layers higher up the tower are less likely to have to pay out claims because the other underwriters will be liable before any claim gets to them. Lower claim risk means cheaper pricing. So the orange line is a curve that flattens out; the higher up the tower you go the cheaper the cost to insure.
Large companies with plenty of capital often decide to self-insure, sometimes through a captive in-house entity, because the bottom layers of the tower can be expensive. The decision for large multinationals hinges around how their cost of capital compares with the cost of insurance. At a certain point up the tower, marked by a pink circle in the diagram, it makes sense to start buying insurance because the costs from that point on are relatively cheap.