diff --git a/notifications/event/event.go b/notifications/event/event.go
index 2ac57ae1..3003820e 100644
--- a/notifications/event/event.go
+++ b/notifications/event/event.go
@@ -27,9 +27,6 @@ type Event struct {
 	// For hosts: {"host": "host_name"} and for services: {"host": "host_name", "service": "service_name"}.
 	Tags map[string]string `json:"tags"`
 
-	// ExtraTags supplement Tags, for example with host or service groups for an Icinga DB source.
-	ExtraTags map[string]string `json:"extra_tags"`
-
 	// Type indicates the type of the event.
 	Type Type `json:"type"`
 	// Severity of the event.
@@ -53,7 +50,20 @@ type Event struct {
 	// from the encoded JSON.
 	MuteReason string `json:"mute_reason,omitempty"`
 
-	// RulesVersion and RuleIds are the source rules matching for this Event.
-	RulesVersion string   `json:"rules_version"`
-	RuleIds      []string `json:"rule_ids"`
+	// CompleteRelations contains a list of relations that should be considered complete for this event.
+	//
+	// Typically, when Icinga Notifications determines that the event didn't provide complete information
+	// needed to evaluate the rules in the [Relations] field, it will instruct the source to provide the
+	// missing information in a follow-up request. This field can be used by the source to indicate that
+	// it already provided all the available information it has for the certain relations and that the
+	// Icinga Notifications should not ask for more information for these relations.
+	CompleteRelations []string `json:"complete_relations,omitempty"`
+
+	// Relations contains additional information about the relations of the object this event is referring to.
+	//
+	// This will be used to evaluate JSONPath expressions in the filter columns of the event rules.
+	// The structure of this field is flexible and can contain any information that might be relevant
+	// for the evaluation of the rules. It will not be used for anything else than evaluating the rules
+	// and will not be persisted to the database as well.
+	Relations map[string]any `json:"relations,omitempty"`
 }
diff --git a/notifications/event/event_test.go b/notifications/event/event_test.go
index 1dcb68a4..99a44d24 100644
--- a/notifications/event/event_test.go
+++ b/notifications/event/event_test.go
@@ -18,16 +18,18 @@ func TestEvent(t *testing.T) {
 			t.Parallel()
 
 			event := &Event{
-				Name:         "TestEvent",
-				URL:          "/icingadb/service?name=https%20ssl%20v3.0%20compatibility%20IE%206.0&host.name=example%20host",
-				Tags:         map[string]string{"tag1": "value1"},
-				ExtraTags:    map[string]string{},
-				Type:         TypeState,
-				Severity:     SeverityOK,
-				Username:     "testuser",
-				Message:      "Test",
-				RulesVersion: "0x1",
-				RuleIds:      []string{"1", "2", "3", "6"},
+				Name:              "TestEvent",
+				URL:               "/icingadb/service?name=https%20ssl%20v3.0%20compatibility%20IE%206.0&host.name=example%20host",
+				Tags:              map[string]string{"tag1": "value1"},
+				Type:              TypeState,
+				Severity:          SeverityOK,
+				Username:          "testuser",
+				Message:           "Test",
+				CompleteRelations: []string{"relation1", "relation2"},
+				Relations: map[string]any{
+					"relation1": "relation1",
+					"relation2": "relation2",
+				},
 			}
 
 			data, err := json.Marshal(event)
@@ -38,13 +40,12 @@ func TestEvent(t *testing.T) {
 					"name":"TestEvent",
 					"url":"/icingadb/service?name=https%20ssl%20v3.0%20compatibility%20IE%206.0&host.name=example%20host",
 					"tags":{"tag1":"value1"},
-					"extra_tags":{},
 					"type":"state",
 					"severity":"ok",
 					"username":"testuser",
 					"message":"Test",
-					"rules_version": "0x1",
-					"rule_ids": ["1", "2", "3", "6"]
+					"complete_relations":["relation1", "relation2"],
+					"relations":{"relation1":"relation1","relation2":"relation2"}
 				}`
 			assert.JSONEq(t, expected, string(data), "JSON encoding does not match expected output")
 		})
diff --git a/notifications/http_headers.go b/notifications/http_headers.go
new file mode 100644
index 00000000..dc8fd346
--- /dev/null
+++ b/notifications/http_headers.go
@@ -0,0 +1,10 @@
+package notifications
+
+// XIcingaRejectIfRelationsIncomplete is a custom HTTP header that can be used by sources to indicate that Icinga
+// Notifications should reject the request if the event's relations are incomplete.
+//
+// By default, Icinga Notifications will attempt to process events even if their relations are incomplete,
+// which may not be desirable in some cases. If a source sets this header to "true", Icinga Notifications
+// rejects the request with all the missing relations in the response body, allowing the source to retry
+// with the complete set of relations.
+const XIcingaRejectIfRelationsIncomplete = "X-Icinga-Reject-If-Relations-Incomplete"