Or like in, and sorry for the dull reference, Warhammer 40k- an Invulnerable save (in this case the Barrier effect) would stack with an Armor save, but not with a Cover save. This is not to save the Invul is always used over Cover or vice versa, just whichever is higher. If you're standing behind a boulder, the rock would prevent the spell effect from ever being taken into account. On the other hand if you're holding up a twig, the Barrier would have a little more of a go at stopping those bullets.
Of course my SA book is still in the mail (and for some time now) so I have no relevant on hands knowledge. Just filling the gap with an educated guess (elongated as always) before the rules lawyers break out.
edit: what's a syntax?