Hi Rudi,
For a detailed understanding on how to create new custom types you can take a look at:- http://wiki.alfresco.com/wiki/Step-By-Step:_Creating_A_Custom_Model
Also find below an example of custom Content Model (OrganizationModel.xml).
Example creates a new type "employee". This new type derives from the existing cm:folder type in Content Services (<parent>cm:folder</parent>). In case you want the objects of your custom types to be visible in the content space UI you should derive them from either cm:folder or cm:cmcontent. Also, this type defines 2 properties namely employeeCode and designation both of them of text type.
To deploy this custom type
1. Go to ContentServices UI @ http://<LCServer>:<Port>/contentspace .
2. Browse to Company Home -> Data Dictionary -> Models.
3. Add Content.
4. Make sure Modify all properties when page closes is checked.
5. Select Activate Model.

Once your model is activated you can see it from Workbench in the list of nodeTypes for operations like StoreContent (see image).
Here is the text of OrganizationModel.xml for your ease.
<model name="org:OrganizationModel" xmlns="http://www.alfresco.org/model/dictionary/1.0">
<imports>
<import uri="http://www.alfresco.org/model/dictionary/1.0" prefix="d"/>
<import uri="http://www.alfresco.org/model/content/1.0" prefix="cm"/>
<import uri="http://www.alfresco.org/model/system/1.0" prefix="sys"/>
</imports>
<!--Introduction of new namespaces defined by this model -->
<namespaces>
<namespace uri="http://www.adobe.com/lc/organization/1.0" prefix="org" />
</namespaces>
<data-types/>
<constraints/>
<types>
<!--<type name="org:company">
<parent>cm:folder</parent>
<archive>false</archive>
<properties>
<property name="org:CompanyName">
<title>CompanyName</title>
<description>CompanyName</description>
<type>d:text</type>
<mandatory>true</mandatory>
<multiple>false</multiple>
<index enabled="true">
<atomic>true</atomic>
</index>
<constraints/>
</property>
<property name="org:cmmLevel">
<title>cmmLevel</title>
<description>cmmLevel</description>
<type>d:text</type>
<mandatory>true</mandatory>
<multiple>false</multiple>
</property>
</properties>
<associations/>
<overrides/>
<mandatory-aspects/>
</type>-->
<type name="org:employee">
<parent>cm:folder</parent>
<archive>false</archive>
<properties>
<property name="org:employeeCode">
<title>employeeCode</title>
<description>employeeCode</description>
<type>d:text</type>
<mandatory>true</mandatory>
<multiple>false</multiple>
<index enabled="true">
<atomic>true</atomic>
</index>
<constraints/>
</property>
<property name="org:designation">
<title>designation</title>
<description>designation</description>
<type>d:text</type>
<mandatory>true</mandatory>
<multiple>true</multiple>
</property>
</properties>
<associations>
<!--<association name="org:employedBy">
<target>
<class>org:company</class>
<mandatory>true</mandatory>
<many>false</many>
</target>
</association>-->
</associations>
<overrides/>
<mandatory-aspects/>
</type>
</types>
<aspects/>
</model>