Please explain "source and sink analysis"

Please explain "source and sink analysis" more briefly that how it helps in our documents or in software engineering ? 

In requirement analysis, first step is source and sink analysis. Source is the origin of any requirement. Source analysis involves the tracing requirements back to origin that who is involved in its initiation.It can be any stakeholder, system or external entity. Since when requirement is originating, there must be an end point for it that where it will be consumed or used. This is sink of requirement.  Source and sink analysis provides us mapping requirements. If some source or sink is missing, it means either some requirement is missing or redundant. It is first step towards completeness of requirements. Documenting it not only aid us in identifying missing requirements but also provide a complete view of a requirement.i.e, from where it will  start and where it will be consumed.