What, exactly does the usrmerge package do?

If /usr is already in root, and not mounted from elsewhere, is the
usrmerge package still required?


From the Devuan announcement on usrmerge:

    Navigating the merged usr transition

 1. Daedalus/stable is the final Devuan release that will function
    without merged usr.
 2. The safest way to migrate from Daedalus/stable to Excalibur/testing
    is to first ensure there is sufficient disk space to accommodate the
    merge, and that you have made backups before proceeding.
 3. Only then install the |usrmerge| package */before/* proceeding with
    the normal dist-upgrade steps.
 4. This document will be updated to reflect future changes as needed.

Does #2, above mean to make sure there is enough space in / to copy in
all of an externally mounted /usr?


I am just trying to clarify what is actually going to happen before it
is suddenly upon me.


Marc