Upgradepkg --root Syntax and Options

I have several systems that serve as upgrade level tests, backups (in addition to actual backup for my Main) and general comparison as well as job specific configs. One of my test systems is trashed from a stupid config error mistake I made with slackpkg so I'd like to explore the --root options... like

Code:upgradepkg --install-new --root otherrootand before I do since "otherroot" isn't well defined in "man installpkg" I need to know if it is

1) OK to do "--root /dev/sdfoo"
or
2) if some other specification or syntax is required
or
3) is some form of chroot required first