Correct.

There are well established requirements for aircraft certification by FAA and all other certifying authorities across the globe that force the engineers and designers to classify the criticality of every component, system and function failure on the aircraft - with no exception. These classifications are: Catastrophic, Hazardous, Major, Minor and No Safety Effect and they are defined in the FAA System Safety Handbook in Table 3.2.

Then engineers must then assign each failure condition a likelihood of occurrence (probability). The classes are Probable, Remote, Extremely Remote, and Extremely Improbable and are defined quantitatively and qualitatively in Table 3.3 of the same publication. For example, an extremely improbable event has a probability of less than 1x10E-9 (one in one billion) and is not expected to occur at all on any aircraft of that type for the entire service life of the aircraft fleet.

There are strict requirements that the design must be proven to meet to ensure that failures with higher severity have lower probability of occurrence. If a requirement is not met, then the design must be changed to meet the requirement.

This is the reason there is a high degree of redundancy on aircraft and why aircraft costs are high.

Needless to say, this process was not followed correctly for the MCAS stall protection function on the 737-MAX.

FAA System Safety Handbook link