Squash, fast-forward only (--squash --ff-only): If the source branch is out of date with the target branch, reject the merge request. Otherwise, combine all commits into one new non-merge commit on the target branch

Use case :
A pull request rebased on master branch contains one feature commit plus some code improvement introduced by the review process.

This give us the Following configuration :

#!git

                   D---D'---D''  Feature-XYZ
                 /
    A---B---C  master

that should be merged as

#!git

    A---B---C ---D master

Latest update from Atlassian 2024-09-30

We've added support for the following additional merge strategies:

• Rebase + fast forward (BCLOUD-16610)
• Rebase + merge (BCLOUD-17881, BCLOUD-19186)
• Squash, fast forward only (BCLOUD-16338)

It's also now possible to sync a feature branch via rebase (BCLOUD-6202).

https://bitbucket.org/blog/branch-sync-merge-rebase