What is the ExtensionContext and its Store used for in JUnit 6?

// ExtensionContext.Store: share data between callbacks within an extension
public class TimingExtension
        implements BeforeTestExecutionCallback, AfterTestExecutionCallback {

    // Namespace prevents collisions between multiple extensions
    private static final Namespace NS =
        Namespace.create(TimingExtension.class);

    @Override
    public void beforeTestExecution(ExtensionContext ctx) {
        // Store the start time under this test's context
        getStore(ctx).put("startTime", System.currentTimeMillis());
    }

    @Override
    public void afterTestExecution(ExtensionContext ctx) {
        long startTime = getStore(ctx).remove("startTime", long.class);
        long elapsed   = System.currentTimeMillis() - startTime;
        System.out.printf("%s took %d ms%n", ctx.getDisplayName(), elapsed);
    }

    private ExtensionContext.Store getStore(ExtensionContext ctx) {
        return ctx.getStore(NS);
    }
}

// ExtensionContext key methods:
// ctx.getDisplayName()           -- test display name
// ctx.getRequiredTestMethod()    -- the @Test Method object
// ctx.getRequiredTestClass()     -- the test class
// ctx.getTags()                  -- Set<String> of @Tag values
// ctx.getTestInstance()          -- Optional<Object> test instance
// ctx.getCancellationToken()     -- JUnit 6: cooperative cancellation
// ctx.getStore(namespace)        -- scoped key-value store
// ctx.getRoot()                  -- root context (global scope)