What are the Extension Model and extension points in JUnit 6?

// Custom extension: time each test and log slow ones
@Target({ElementType.TYPE, ElementType.METHOD})
@Retention(RetentionPolicy.RUNTIME)
@ExtendWith(TimingExtension.class)
public @interface Timed { long thresholdMs() default 100; }

public class TimingExtension
        implements BeforeTestExecutionCallback, AfterTestExecutionCallback {

    private static final String START = "start_time";

    @Override
    public void beforeTestExecution(ExtensionContext ctx) {
        ctx.getStore(GLOBAL).put(START, System.currentTimeMillis());
    }

    @Override
    public void afterTestExecution(ExtensionContext ctx) {
        long start = ctx.getStore(GLOBAL).remove(START, long.class);
        long elapsed = System.currentTimeMillis() - start;
        Timed ann = ctx.getRequiredTestMethod().getAnnotation(Timed.class);
        if (ann != null && elapsed > ann.thresholdMs()) {
            System.out.printf("[SLOW] %s took %dms%n",
                ctx.getDisplayName(), elapsed);
        }
    }
}

// Usage:
@Test
@Timed(thresholdMs = 200)
void slowIntegrationTest() { ... }