A user story is a short sentence of the form: As a role, I want to do something in order to create an outcome
User stories are focused on the roles and the outcomes.
Each user story will require one or more capabilities in order to fulfill the user story. Many times the same capabilities will be part of many user stories.
Examining the big picture of capabilities across all user stories will show the overall capability map required to fulfill user stories. The overall view allows you to make better decisions about architecting the capabilities.