Only the Developer decides whether she/he will be assigned to a particular User Story, thus including it in the Sprint Backlog.
In doing so, the Developer is committing to delivering the particular User Story. (More about commitment can be found here.) They know best what they are capable of.
But this does not mean that this Developer is solely responsible for delivering the User Story. The whole team is responsible.
Also, more than one Developer can be assigned to a User Story. You will read about that in an upcoming article.
Nobody can force the Developers to accept more work than they choose to.
Each Developer is aware of their own capacity so they are the ones who can say how much work they can manage during a Sprint. If someone tries to get the Developer to take more work into the Sprint, the Scrum Master needs to step in.
By selecting a User Story, its Developers commit to its completion. That means that they will do everything in their power to finish the User Story in that Sprint, but is not an absolute guarantee that the User Story will be delivered.
We already wrote about this in previous articles (e.g. A Developer’s To Do List). Nevertheless, there is one important point in this rule: there is no absolute guarantee that the User Story will be delivered.
While working on the User Story, the Developers may realize that it is bigger than they thought. Or, they find dependencies that they cannot influence, such as from other team members. So, the team, Scrum Master and Product Owner must work together to help remove the impediments. If anything jeopardizes finishing the User Story, the Product Owner has to be informed immediately, so they can make strategic decisions based on the scope and priorities.
We hope you found some new and useful information in this article. Next time, we will explain why multiple Developers can work on one User Story while only a single Developer can work on a Task.