In insurance, stacking refers to tapping multiple limits, coverage parts, or policies for a single loss. It is relevant when one source of insurance is insufficient.
This comes up a lot in large environmental claims. A factory may have consistently had what was considered sufficient limits through the 60s and 70s. During that time period, environmental knowledge and protections were quite limited. Years later it comes to light that the company’s waste caused environmental catastrophe. Now the company faces damages in the tens of millions. None of its policies will, alone, come close to covering the liability. But, in “continuous trigger” jurisdictions (where the loss is deemed to continuously occur, as opposed to occurring at a particular time), many policies across those years will be triggered and, as such, may have to pay out up to the limits. This is one example of stacking.