Circuit Balances:

Of main importance within a production cascade is the ability to calculate pressures, temperatures, and flows of the procedure gas that cannot be practically or economically measured through instrumentation. Because the cascade is made up of sections each of that holds a large number of identical stages and because the stage is the smallest separating unit of a cascade, those calculations are made on a stage basis. This system of calculating pressures, temperatures, and flows is generally referred to as a circuit balance and consists of a series of calculations made from those pressures and temperatures ordinarily measured through instrumentation within the cascade, and from experimental data acquire from test loop studies.

For description reasons, the pressures, temperatures, and flows in a classical X-31 stage that are taken through instrumentation, plus those to be calculated.  It is obvious in which the cost to install instruments within every stage to measure all of the values denoted would be prohibitive.