Basically, a screen-flow can be used when you have a single user to fill multiple pages, forms or screens. 

Like filling a credit card application with Personal details, Work details, Background Information etc.

Screen flow presents a sequence of assignments to one user. You can use a screen flow to supplement the workflow in a case by calling it from the Subprocess shape in another flow. 

The following features distinguish screen flows from other types of flows:

• The ability to jump ahead or go back to different entry points in the flow at run time. 
• Routing options that are configured in the Start shape instead of individual assignments since entire flow should be routed to single user
• A single harness that defines the presentation, such as tabs or nodes in a tree, for every step in the screen flow.
• When considering a screen flow in your application, keep in mind that not all shapes are supported and that a screen flow cannot be designated as a starter flow. 

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