Which branch must be integrated into the mainline immediately after feature stabilization?

The integration of a branch into the mainline immediately after feature stabilization is a best practice in continuous integration and release management. A release branch is specifically meant for preparing new versions of the software for deployment. Once the features have been stabilized—meaning they have been tested and approved for production—it's essential to merge this branch into the mainline to ensure that all new code and fixes are included in the upcoming release.

This ensures that the mainline remains up to date and reflects the latest state of the project, thus minimizing the potential for conflicts in the future. By integrating the release branch promptly, the development team can streamline the process of software delivery, allowing subsequent deployment activities to commence smoothly. This practice helps in maintaining a reliable and efficient workflow, particularly in agile environments where iterations and rapid releases are common.

In contrast, the other types of branches serve different specific purposes within the development lifecycle. User story branches typically focus on individual functionality and might require additional testing before integration. Defect branches are used for fixing specific bugs and might not involve complete feature sets, and hotfix branches are created for urgent fixes in production but don't necessarily represent a full release cycle. Integrating these branches does not directly correspond with the stable release process like a release branch does.