Work product slots are indirections for the inputs of tasks of a Practice that allow practices to be documented independent of any other practice, i.e. independent of the work products produced by other practices. Practice task refer to work product slots as inputs, rather than refer directly to specific work products. Work product slots should never be listed as an output of a task. 

Practices that create specific work products indicate what work product slots those concrete work products fulfill. Then, when both the slot fulfilling practices and the slot-consuming practices are included in a configuration, the fulfilling work products will be associated with the work product slots.

For example, it is quite possible to write a task for creating an object-oriented analysis model for a set of requirements independent of the concrete format the requirements have been captured in and the practice that was used to elicit them. The requirements could be use cases, scenarios, user stories, stakeholder requests, an IEEE-830 compliant specification or any other form. Hence, you could model an input relationship to a work product slot such as specification and describe for the analysis task how one would look for specific properties in the input specification work product slot, such as the description of a user interaction or the description of a static structure, to denote which parts of the specification are relevant to consider for which parts of the analysis.  On the other hand, when a specific practice is selected for capturing and representing a specification such as Use-Case Based Development, and is combined with the object-oriented analysis practice, you actually want to see the concrete work products available through the selection of that use case practice, substituting the abstract specification placeholder when reviewing the analysis task. In addition, you will expect that use-case specific guidance to be available for the use case based analysis as well, refining the generic instruction of the task.  Work product slots support this.