diff --git a/source/development/frontend/models.rst b/source/development/frontend/models.rst
index a669fef..7ca4829 100644
--- a/source/development/frontend/models.rst
+++ b/source/development/frontend/models.rst
@@ -20,4 +20,5 @@ In this chapter we will explain how models are designed and build.
    models_example
    models_guidelines
    models_fieldtypes
+   models_customfields
    models_constraints
diff --git a/source/development/frontend/models_customfields.rst b/source/development/frontend/models_customfields.rst
new file mode 100644
index 0000000..06dea25
--- /dev/null
+++ b/source/development/frontend/models_customfields.rst
@@ -0,0 +1,72 @@
+-----------------------------------
+Custom (app specific) field types
+-----------------------------------
+
+Applications can add their own custom field types, which should be derived from :code:`BaseField` or one of its descendants.
+A very simple single item custom field type could look like this:
+
+Build the field type
+........................................
+
+.. code-block:: php
+
+    <?php
+    namespace myVendorName\myModule;
+    use OPNsense\Base\FieldTypes\BaseField;
+    use Phalcon\Validation\Validator\Regex;
+     
+    class SimpleCustomField extends BaseField
+    {
+        protected $internalIsContainer = false;
+        protected $internalValidationMessage = "standard error";
+        public function getValidators()
+        {
+            $validators = parent::getValidators();
+            $reservedwords = array('all', 'pass', 'block', 'out');
+            $validators[] = new ExclusionIn(array(
+                'message' => 'can not use a reserved word',
+                'domain' => $reservedwords));
+        }
+        return $validators;
+    }
+
+This example extends the standard validations with a list of reserved words, in which case it would yield :code:`can not use a reserved word`
+if one of the reserved words are provided.
+
+.. Note::
+
+    This file should be placed in the subdirectory :code:`FieldTypes` of the model itself.
+
+.. Tip::
+
+    Use :code:`BaseListField` as simple template for list type items.
+
+
+Use in model
+.................
+
+The validation can be used as any standard type, but using :code:`.\` the model knows it concerns a local field.
+
+.. code-block:: xml
+
+    <model>
+        <mount>//OPNsense/MyFirst/App</mount>
+        <version>1.0.0</version>
+        <description>
+            My first application
+        </description>
+        <items>
+            <general>
+                <name type=".\SimpleCustomField">
+                    <Required>Y</Required>
+                </name>
+            </general>
+        </items>
+    </model>
+
+
+.. Tip::
+
+    Inspect the `basic field <https://github.com/opnsense/core/tree/master/src/opnsense/mvc/app/models/OPNsense/Base/FieldTypes>`__ types
+    for inspiration, a concrete example of a custom field type can be found in the
+    `firewall <https://github.com/opnsense/core/tree/master/src/opnsense/mvc/app/models/OPNsense/Firewall>`__ section