diff --git a/doc/problem_definition.rst b/doc/problem_definition.rst
index a9a52b6..ed30681 100644
--- a/doc/problem_definition.rst
+++ b/doc/problem_definition.rst
@@ -118,10 +118,13 @@ can be specified like
   selected model.
 
 Here, the format for a single model is shown. Multiple models can be specified
-as a YAML list of the same format. The only required key is the ``petab_yaml``,
-as a model requires a PEtab problem. Other keys are required in different
+as a YAML list of the same format. Some optional keys are required in different
 contexts (for example, model comparison will require ``criteria``).
 
+Brief format description
+^^^^^^^^^^^^^^^^^^^^^^^^
+
+
 .. code-block:: yaml
 
    model_subspace_id: # str (required).
@@ -146,5 +149,8 @@ contexts (for example, model comparison will require ``criteria``).
 - ``parameters``: The parameter combination from the model space file that defines this model (either values or ``"estimate"``). Not the calibrated values, which are in ``estimated_parameters``!
 - ``predecessor_model_hash``: The hash of the model that preceded this model during the model selection process. Will be ``virtual_initial_model-`` if the model had no predecessor model.
 
+Schema
+^^^^^^
+
 .. literalinclude:: standard/model.yaml
    :language: yaml