What exactly is a "grafted" commit in a shallow clone?

When doing a shallow clone in git (using the --depth option), the root commit is marked as grafted.

Googling didn't lead to any satisfacting documentation.

It doesn't seem to have anything to do with git grafts, which the similar terminology would imply.
Is it just a flag to signal that this commit actually has more parents and isn't the "real" root commit? Or is there something more special about it?

Solution 1:

From your link:

It works by letting users record fake ancestry information for commits. This way you can make git pretend the set of parents a commit has is different from what was recorded when the commit was created.

In a shallow clone, your root commit is one that should have parents, but not in your repo. So it seems a good use case for grafting.

In effect:

Def.: Shallow commits do have parents, but not in the shallow repo, and therefore grafts are introduced pretending that these commits have no parents.