undocumented git pack-objects --unpacked=file

A child process of "git repack -a -d -f" was doing:

  PID TTY      STAT   TIME COMMAND
20178 pts/3    R+   491:31 git pack-objects --non-empty --all --reflog --unpacked=pack-41dbbd4c9b63110b8122f9d78a15668aa6b2b273.pack

The git-pack-objects(1) manual page describes:

| git-pack-objects [-q] [--no-reuse-delta] [--delta-base-offset] [--non-empty]
|         [--local] [--incremental] [--window=N] [--depth=N] [--all-progress]
|         [--revs [--unpacked | --all]*] [--stdout | base-name] < object-list
...
| --unpacked
|         This implies --revs. When processing the list of revision
|         arguments read from the standard input, limit the objects
|         packed to those that are not already packed.

However, it does not say what
--unpacked=pack-41dbbd4c9b63110b8122f9d78a15668aa6b2b273.pack
might mean.  grep unpacked= Documentation/* did not find anything
either.  Could you please document the meaning of this option?