diff --git a/README.md b/README.md
index 6f4f9297..e552357f 100644
--- a/README.md
+++ b/README.md
@@ -360,7 +360,6 @@ The following tests are not yet implemented and therefore missing:
 
 - Informative Test 6.2.13
 - Informative Test 6.2.14
-- Informative Test 6.2.15
 - Informative Test 6.2.16
 - Informative Test 6.2.17
 
@@ -480,6 +479,7 @@ export const informativeTest_6_3_9: DocumentTest
 export const informativeTest_6_3_10: DocumentTest
 export const informativeTest_6_3_11: DocumentTest
 export const informativeTest_6_3_12: DocumentTest
+export const informativeTest_6_3_15: DocumentTest
 ```
 
 [(back to top)](#bsi-csaf-validator-lib)
diff --git a/csaf_2_1/informativeTests.js b/csaf_2_1/informativeTests.js
index b3142316..28f11615 100644
--- a/csaf_2_1/informativeTests.js
+++ b/csaf_2_1/informativeTests.js
@@ -12,3 +12,4 @@ export { informativeTest_6_3_1 } from './informativeTests/informativeTest_6_3_1.
 export { informativeTest_6_3_2 } from './informativeTests/informativeTest_6_3_2.js'
 export { informativeTest_6_3_4 } from './informativeTests/informativeTest_6_3_4.js'
 export { informativeTest_6_3_12 } from './informativeTests/informativeTest_6_3_12.js'
+export { informativeTest_6_3_15 } from './informativeTests/informativeTest_6_3_15.js'
diff --git a/csaf_2_1/informativeTests/informativeTest_6_3_15.js b/csaf_2_1/informativeTests/informativeTest_6_3_15.js
new file mode 100644
index 00000000..dfc306fb
--- /dev/null
+++ b/csaf_2_1/informativeTests/informativeTest_6_3_15.js
@@ -0,0 +1,134 @@
+import Ajv from 'ajv/dist/jtd.js'
+
+const ajv = new Ajv()
+
+const PREFIX_EXTENSION_NAMESPACE = 'x_'
+
+/**
+ * @typedef {object} Selection
+ * @property {string} [name]
+ * @property {string} [namespace]
+ * @property {string} [version]
+ */
+
+/**
+ * @typedef {object} Ssvc2
+ * @property {Array<Selection>} [selections]
+ */
+
+/**
+ * @typedef {object} MetricContent
+ * @property {Ssvc2} [ssvc_v2]
+ */
+
+/**
+ * @typedef {object} Metric
+ * @property {MetricContent} [content]
+ */
+
+const inputSchema = /** @type {const} */ ({
+  additionalProperties: true,
+  properties: {
+    document: {
+      additionalProperties: true,
+      properties: {
+        distribution: {
+          additionalProperties: true,
+          properties: {
+            tlp: {
+              additionalProperties: true,
+              properties: {
+                label: { type: 'string' },
+              },
+            },
+          },
+        },
+      },
+    },
+    vulnerabilities: {
+      elements: {
+        additionalProperties: true,
+        properties: {},
+        optionalProperties: {
+          metrics: {
+            elements: {
+              additionalProperties: true,
+              optionalProperties: {
+                content: {
+                  additionalProperties: true,
+                  optionalProperties: {
+                    ssvc_v2: {
+                      additionalProperties: true,
+                      optionalProperties: {
+                        selections: {
+                          elements: {
+                            additionalProperties: true,
+                            optionalProperties: {
+                              name: { type: 'string' },
+                              namespace: { type: 'string' },
+                              version: { type: 'string' },
+                            },
+                          },
+                        },
+                      },
+                    },
+                  },
+                },
+              },
+            },
+          },
+        },
+      },
+    },
+  },
+})
+
+const validateInput = ajv.compile(inputSchema)
+
+/**
+ * @param {string | undefined } namespace
+ */
+function namespaceUsesExtension(namespace) {
+  return namespace ? namespace.startsWith(PREFIX_EXTENSION_NAMESPACE) : false
+}
+
+/**
+ * For each SSVC decision point given under `selections`, it MUST be tested that the `namespace` does not use an extension
+ * if the document is not labeled `TLP:CLEAR`.
+ * Namespaces reserved for special purpose MUST be treated as per their definition.
+ * @param {unknown} doc
+ * @returns
+ */
+export function informativeTest_6_3_15(doc) {
+  const ctx = {
+    infos: /** @type {Array<{ message: string; instancePath: string }>} */ ([]),
+  }
+
+  if (!validateInput(doc)) {
+    return ctx
+  }
+
+  const vulnerabilities = doc.vulnerabilities
+
+  vulnerabilities.forEach((vulnerability, vulnerabilityIndex) => {
+    /** @type {Array<Metric> | undefined} */
+    const metrics = vulnerability.metrics
+    metrics?.forEach((metric, metricIndex) => {
+      const selections = metric?.content?.ssvc_v2?.selections
+      selections?.forEach((selection, selectionIndex) => {
+        if (
+          namespaceUsesExtension(selection.namespace) &&
+          doc.document.distribution.tlp.label !== 'TLP:CLEAR'
+        ) {
+          ctx.infos.push({
+            instancePath: `/vulnerabilities/${vulnerabilityIndex}/metrics/${metricIndex}/content/ssvc_v2/selections/${selectionIndex}/namespace`,
+            message:
+              'namespace uses an extension and document is not labeled TLP:CLEAR',
+          })
+        }
+      })
+    })
+  })
+
+  return ctx
+}
diff --git a/tests/csaf_2_1/informativeTest_6_3_15.js b/tests/csaf_2_1/informativeTest_6_3_15.js
new file mode 100644
index 00000000..a87fd4d5
--- /dev/null
+++ b/tests/csaf_2_1/informativeTest_6_3_15.js
@@ -0,0 +1,53 @@
+import assert from 'node:assert'
+import { informativeTest_6_3_15 } from '../../csaf_2_1/informativeTests.js'
+
+describe('informativeTest_6_3_15', function () {
+  it('only runs on relevant documents', function () {
+    assert.equal(informativeTest_6_3_15({ document: 'mydoc' }).infos.length, 0)
+  })
+  it('test input schema with not considered json object in vulnerabilities', function () {
+    assert.equal(
+      informativeTest_6_3_15({
+        document: {
+          distribution: {
+            tlp: {
+              label: 'GREEN',
+            },
+          },
+        },
+        vulnerabilities: [
+          {},
+          {
+            cve: 'CVE-1900-0001',
+            metrics: [
+              {
+                content: {
+                  ssvc_v2: {
+                    id: 'CVE-1900-0001',
+                    schemaVersion: '1-0-1',
+                    selections: [
+                      {
+                        name: 'Technical Impact',
+                        namespace: 'x_additional-technical-impacts',
+                        values: ['Total'],
+                        version: '1.0.0',
+                      },
+                      {
+                        name: 'Technical Impact',
+                        values: ['Total'],
+                        version: '1.0.0',
+                      },
+                    ],
+                    timestamp: '2024-01-24T10:00:00.000Z',
+                  },
+                },
+                products: ['CSAFPID-9080700'],
+              },
+            ],
+          },
+        ],
+      }).infos.length,
+      1
+    )
+  })
+})
diff --git a/tests/csaf_2_1/oasis.js b/tests/csaf_2_1/oasis.js
index 0e9d2e60..5b621df7 100644
--- a/tests/csaf_2_1/oasis.js
+++ b/tests/csaf_2_1/oasis.js
@@ -62,7 +62,6 @@ const excluded = [
   '6.3.12',
   '6.3.13',
   '6.3.14',
-  '6.3.15',
   '6.3.16',
   '6.3.17',
   '6.3.18',