diff --git a/api/src/main/kotlin/dev/bpmcrafters/processengineapi/CommonRestrictions.kt b/api/src/main/kotlin/dev/bpmcrafters/processengineapi/CommonRestrictions.kt
index 2043f35..11c1d05 100644
--- a/api/src/main/kotlin/dev/bpmcrafters/processengineapi/CommonRestrictions.kt
+++ b/api/src/main/kotlin/dev/bpmcrafters/processengineapi/CommonRestrictions.kt
@@ -28,6 +28,10 @@ object CommonRestrictions {
 
   /**
    * Id provided by the runtime identifying the process instance.
+   *
+   * In the meta of a task, the value is the same as
+   * [dev.bpmcrafters.processengineapi.process.ProcessInformation.instanceId] returned when that process
+   * instance was started. An engine which uses a different value in one of the two places has a bug.
    */
   const val PROCESS_INSTANCE_ID = "processInstanceId"
 
diff --git a/api/src/main/kotlin/dev/bpmcrafters/processengineapi/process/ProcessInformation.kt b/api/src/main/kotlin/dev/bpmcrafters/processengineapi/process/ProcessInformation.kt
index eb0ee5f..fa7c749 100644
--- a/api/src/main/kotlin/dev/bpmcrafters/processengineapi/process/ProcessInformation.kt
+++ b/api/src/main/kotlin/dev/bpmcrafters/processengineapi/process/ProcessInformation.kt
@@ -7,6 +7,11 @@ package dev.bpmcrafters.processengineapi.process
 data class ProcessInformation(
   /**
    * Reference to the instance.
+   *
+   * When an engine puts [dev.bpmcrafters.processengineapi.CommonRestrictions.PROCESS_INSTANCE_ID] into the
+   * meta of a task of this instance, it is the same value. A caller can rely on that to match a delivered
+   * task to the process instance it started. An engine which uses a different value in one of the two places
+   * has a bug.
    */
   val instanceId: String,
   /**