Test Strategy Altitude
Deciding which risks are verified at which level, so that each layer tests something the layers below it structurally cannot.
A testing strategy is an allocation of risk to levels. The question for each layer is not "what can we test here?" — almost anything can be tested anywhere — but "what can only be tested here, and what is cheapest to test here?".
Unit tests are the only place where a branch buried in a rarely reached code path is cheap to reach. Integration tests are the only place where the ORM's actual SQL, the transaction boundary and the real index behaviour appear. Contract tests are the only place where a provider learns it broke a consumer without running that consumer. End-to-end tests are the only place where routing, authentication and configuration meet.
Written this way, the redundancy becomes visible. Most suites test the same business rule at four levels, which is four times the maintenance and one unit of confidence, while the configuration mistake that actually causes outages is tested nowhere. The strategy document is worth having only if it makes that reallocation happen.