Required When date

The “Required When” date is an option to set a specific deadline for when the story or issue is required to be completed. It can be seen as the When part of the user story.

To obtain a more detailed picture of a given deadline, a milestone should be created and linked to the story instead. 

If a Required When date is set, this will be included in deriving the effective deadline.The planning date must meet the effective deadline, otherwise warnings are shown. 

Read more about Deadlines and Planned Date here.