Why placement is the whole game
A thickness reading is only as informative as the location it monitors. Corrosion in piping is rarely uniform: turbulence attacks elbows and tees, injection points see concentrated chemistry, stagnant branches corrode at multiples of the flowing line's rate, and supports and soil-to-air interfaces trap moisture. A CML plan drawn on straight, accessible, healthy runs measures where the pipe is at its best — and certifies a system that fails somewhere else.
What the data feeds
Each CML's history yields a corrosion rate; the worst credible rate and the minimum required thickness yield a remaining life; and remaining life drives the inspection interval and the run-repair-replace decision under the governing in-service code. The chain is only as strong as measurement discipline: same location, documented, repeatable — which is why CMLs are marked and mapped rather than left to the technician's judgment each outage.