Add instruction for BWC tests on distribution level

BUILDING.md

@@ -270,4 +270,4 @@ To exclude add the following in `settings.gradle` file ```startParameter.exclude
 
 The available [OpenSearch plugins](https://opensearch.org/docs/latest/install-and-configure/install-opensearch/plugins/#available-plugins) and [OpenSearch Dashboards plugins](https://opensearch.org/docs/latest/install-and-configure/install-dashboards/plugins/#available-plugins) are listed on the [documentation website](https://opensearch.org/docs/latest/). 
 
-If you create a new plugin or update the name of an existing one, make sure to update the list of plugins. You can either edit the [OpenSearch plugins doc page](https://github.com/opensearch-project/documentation-website/blob/main/_install-and-configure/install-opensearch/plugins.md) or [OpenSearch Dashboards plugins doc page](https://github.com/opensearch-project/documentation-website/blob/main/_install-and-configure/install-dashboards/plugins.md) in the documentation repo, or [open a documentation issue](https://github.com/opensearch-project/documentation-website/issues/new/choose).
+If you create a new plugin or update the name of an existing one, make sure to update the list of plugins. You can either edit the [OpenSearch plugins doc page](https://github.com/opensearch-project/documentation-website/blob/main/_install-and-configure/plugins.md) or [OpenSearch Dashboards plugins doc page](https://github.com/opensearch-project/documentation-website/blob/main/_install-and-configure/install-dashboards/plugins.md) in the documentation repo, or [open a documentation issue](https://github.com/opensearch-project/documentation-website/issues/new/choose).

TESTING.md

@@ -12,6 +12,7 @@
     - [Codecov integration in Pull Requests](#codecov-integration-in-pull-requests)
   - [Backwards Compatibility Testing](#backwards-compatibility-testing)
     - [Types of BWC tests](#types-of-bwc-tests)
+    - [BWC tests on distribution bundle level](#bwc-tests-on-distribution-bundle-level)
     - [Hooking the BWC tests to CI](#hooking-the-bwc-tests-to-ci)
     - [BWC Version test matrix](#bwc-version-test-matrix)
   - [Gradle Plugins](#gradle-plugins)
@@ -98,6 +99,17 @@ See [anomaly-detection#158](https://github.com/opensearch-project/anomaly-detect
 
 Each plugin can test various functionalities in their bwc tests and add more scenarios if needed.
 
+#### BWC tests on distribution bundle level
+
+The BWC tests running on distribution level are using the same framework from OpenSearch. The test cluster is spin up with the `latest` distribution bundle of provided version exclusively when the project is initialized with property `-PcustomDistributionDownloadType=bundle`.
+
+The distribution bundle will be obtained with this format URL: `https://ci.opensearch.org/ci/dbc/distribution-build-opensearch/1.3.2/latest/linux/x64/tar/dist/opensearch/opensearch-1.3.2-linux-x64.tar.gz`
+This URL pattern starts to be supported as far as `v1.3.2` so The distribution bundle set up in the test cluster can be all versions after version `v1.3.2`. 
+
+On CI level for plugins, security certificates need to be manually imported when spinning up the test cluster as security plugin is included in the distribution bundle. When upgrading the version within the test cluster, `nextNodeToNextVersion` is used for a single node upgrade and `goToNextVersion` is for a full restart upgrade.
+
+See [anomaly-detection#766](https://github.com/opensearch-project/anomaly-detection/pull/766) or [observability#1366](https://github.com/opensearch-project/observability/pull/1366) for more information.
+
 #### Hooking the BWC tests to CI
 
 The bwc tests can be hooked to the CI workflow in the plugin to run it with each pull request and push.