<select mo:type="ComboBox" mo:name="${fieldName}" mo:value="${value}"  
     mo:props="nonBlank:${nonBlank}, valHandler:${valHandler}, valErrorHandler:${valErrorHandler}">
    <option value="1">CAT1</option>
    <option value="2">CAT2</option>
  </select>

${fieldName}

The name of the ProcessField

${value}

The initial value of the field

${nonBlank}

If "true" then the field cannot be left blank

${valHandler}

The name of the validation handler function

${valErrorHandler}

The name of the validation error handler function.