Every optimisation in the previous lecture makes the same suite faster. This one questions whether the whole suite should run at all, which is a bigger lever and a harder conversation.
The default nobody chose. Most teams run everything on every commit because that is what the first pipeline did, and nobody revisited it. As the suite grows, that default quietly becomes the reason engineers stop waiting for tests.
Tier by risk and by speed.
Smoke, one to two minutes. Ten to twenty tests. Can users log in, load the main screen, and complete the core action? Runs on every commit, every branch, always. If this fails, nothing else matters.