It's flexible; presubmit tests can be configured per-directory. There's also an option to run all tests of packages that could be affected by a change based on

It's flexible; presubmit tests can be configured per-directory. There's also an option to run all tests of packages that could be affected by a change based on the Blaze dependency graph. If you're making changes to a package with tons of dependencies such as Guava, for a risky change you might want to run all affected tests, but for a minor change you might want to run just the standard unit tests. As a compromise, there's also an option to run a random sample of affected tests.