Multiple measurable criteria for success. For example, given a walk in the woods, primary variables might be fun, safety, low cost, and exercise.
A collection of primary variables should be optimized, should be reinforcing in the long terms, but sometimes they conflict with each other in the short term. Primary variables are often influenced indirectly, such as by tweaking secondary or tertiary variables. Buying good hiking boots might increase safety but add cost. The more expensive boots, the higher the safety but higher the added cost.
The concept of "Primary variables" is often called "balanced score cards" in the consulting world, and built into "systems dynamics" in the type of simulations called "interactive spreadsheets."