Adjust documentation

Signed-off-by: ClemensLinnhoff <[email protected]>

Author: ClemensLinnhoff

doc/architecture/architecture_overview.adoc

@@ -51,7 +51,8 @@ This has been done to allow backward-compatible changes in the field.
 Additionally, this is the default behavior in Protocol Buffer version 3 that no longer has the `required` type.
 Setting all fields to `optional` thus ensures update compatibility.
 However, this does not mean that it is optional to fill the field.
-For the purpose of providing a complete interface, all existing fields should be set, unless not setting a field carries a specific meaning, as indicated in the accompanying comment.
+Fields with the rule `is_set` are mandatory and have to be set at all times.
+All other fields have to be set according to the requirements of the connected models.
 
 NOTE: All field numbers equal to or greater than 10000 are available for user-specific extensions via custom fields.
 No future evolution of OSI will therefore use field numbers equal to or greater than 10000.