Concept page:
http://wiki.magnolia-cms.com/display/DEV/Concept+-+New+Categorization+app#Concept-NewCategorizationapp-Field

Create a generic multiselect field.

• Create a basic SelectionField (Input text + choose button)
• Create a MultiSelectField (Set of SelectionField)
• Create the appropriate definition (MultiSelectFieldDefinition)
• Create Specific properties in order to store the multi selected values into 3 different way:
  • Single field property (value separated with string)
  • Multi-property values
  • Multi sub nodes