Work Product Slots when are defined when you need to pass information between Practices.

Work product slots are defined in Core plug-ins where their definition can be shared. For more information on the practice library plug-in types, see Concept: Practice Library Plug-In Types.  For more information on defining plug-ins, see Guideline: Defining Plugins.

Work product slots are implemented as work products. However, they are "special" work products in that they are implementation/representation agnostic. Thus, for general information on defining work products, see Guideline: Defining Method Content Elements.

It is important to remember that work products slots are not “real” elements, in the method sense. Thus, there are some additional authoring guidelines:

• Work product slots may appear as inputs to tasks
• Work product slots may not appear as an output to tasks
• Work product slots are filled by real work products in specific practices  
• Work product slots should not be categorized using standard categories
• When naming separate work product slots, use a descriptive name that indicates explicitly that the element is a slot. Specifically, the slot's presentation name should be between brackets and it's internal name should end in (“_slot”).  Such conventions make it easy to see what is a slot and what is an actual artifact that fills a slot. 
  For example:
• • Presentation Name: [Specified System]
  • Name: specified_system_slot