Github's "squash and merge" merge method - what does "combine" mean?

Context

Github's article on merge-methods states

>Instead of seeing all of a contributor's individual commits from a topic branch, the commits are combined into one commit and merged into the default branch.

The method is illustrated by the following picture, where commit D and E are "combined".

Question

What does the article mean by "combine" commits?

Please correct me if I am wrong, but as far as I understand git, the content of commit D has absolutely no relevance for the content (i.e. working tree) of the result-commit F, no?

Or in other words, in any two cases where commit E has the same content and commit D differs, commit F will look the same, right?

Your mental model seems to be that a commit is a tree of files, not a set of changes, and that's indeed how Git implements it. So in that sense, you are right that only the contents of commit E matter.

However, when viewing a commit as a set of changes, it's easier to see what "combine" means: first the changes in D are applied, then the changes in E, and the resulting set of changes becomes F.

However you view it, though, the commit messages for F will (by default) be the combination of the commit messages of D and E.

The net changes from the squashed commits (D and E in the diagram) become a new single commit (F). Instead of putting two (or three, or four, or however many) incremental commits into main, we now have one new commit in main containing the sum of all changes from that other branch.

How F looks will depend on what happened in the other two:

• if D has changes at the beginning of a file, and E has changes at the end of the same file, then F will be the changes at the beginning and end of that one file
• if D changes one file, and E changes a different one, then F will have changes to both files
• if D has a lot of work, and E is just a typo fix, then F will be all that work but with the fixed typo
• if E overwrites everything in D, then F will just be the contents of E
• etc.