diff --git a/src/Model/Item.php b/src/Model/Item.php
index aff9dfed0b..5047af5986 100644
--- a/src/Model/Item.php
+++ b/src/Model/Item.php
@@ -1773,7 +1773,10 @@ class Item
 	}
 
 	/**
-	 * Creates an unique guid out of a given uri
+	 * Creates an unique guid out of a given uri.
+	 * This function is used for messages outside the fediverse (Connector posts, feeds, Mails, ...)
+	 * Posts that are created on this system are using System::createUUID.
+	 * Received ActivityPub posts are using Processor::getGUIDByURL.
 	 *
 	 * @param string $uri uri of an item entry
 	 * @param string $host hostname for the GUID prefix
diff --git a/src/Protocol/ActivityPub/Processor.php b/src/Protocol/ActivityPub/Processor.php
index 9aeb301cf3..5651343cb5 100644
--- a/src/Protocol/ActivityPub/Processor.php
+++ b/src/Protocol/ActivityPub/Processor.php
@@ -553,7 +553,7 @@ class Processor
 	}
 
 	/**
-	 * Generate a GUID out of an URL
+	 * Generate a GUID out of an URL of an ActivityPub post.
 	 *
 	 * @param string $url message URL
 	 * @return string with GUID