Final answer:
Feature branches are merged into the promotion branch based on manual prioritization by the user, not by an automatic predefined order. The development team or project manager determines the merge order according to the project's needs and workflow rules.
Step-by-step explanation:
By default, user story feature branches are merged into the promotion branch not based on an automatic predefined order such as alphabetical or chronological. Instead, the merge process typically requires manual prioritization by the user. It is the responsibility of the development team or the individual managing the project repository to decide which feature branches get merged into the promotion branch, and when this occurs. This process is often guided by the project's release schedule, priority of the features, or project workflow rules defined by the software development team. Therefore, the merge order is intentional and aligned with the team's needs rather than being left to a random or automatically generated sequence.