Requirement

Instead of maintaining many categories each with a limited set of keywords, the customer wanted to be able to use one category, containing a hierarchy of subcategories, each in turn containing the appropriate keywords.


In the Schema definition, for each field, that uses a category, it should be possible to select one of the subcategories as the root.  
When editing or creating a component based on this schema, the list of keywords should be limited to the keywords which are hierarchically below the configured subcategory.
When the field definition does not have a subcategory defined, it should use the default behavior, to show the full list of keywords for the category.

Implementation

To mimic the requested behavior, keywords are treated as a subcategory. By selecting a keyword, it will be used as the Root element.
This alternative Root is stored with in the schema definition, as part of the field description.
Each field can have its own optional root keyword definition.

The implementation of this functionality consists of two CM GUI extensions.

The Schema Editor contains an additional control which allows the administrator to select a Root keyword.
This control is only visible when "Values will be selected from a List" and "Category" are selected

The Component Editor is extended to be Root Keyword aware and generate and show lists limited by this new root as can be seen in the two screenshots below

Finally here is an example of how the definition is stored in the Schema

<xsd:element name="subCategoryPG1" minOccurs="1" maxOccurs="1" type="category:SuperCategory">
  <xsd:annotation>
    <xsd:appinfo>
      <tcm:ExtensionXml xmlns:tcm="http://www.tridion.com/ContentManager/5.0">
        <RootKeyword xmlns="http://www.tridion.com/ContentManager/5.0" Id="tcm:12-880-1024"></RootKeyword>
      </tcm:ExtensionXml>
      <tcm:Size xmlns:tcm="http://www.tridion.com/ContentManager/5.0">1</tcm:Size>
      <tcm:listtype xmlns:tcm="http://www.tridion.com/ContentManager/5.0">select</tcm:listtype>
    </xsd:appinfo>
  </xsd:annotation>
</xsd:element>