Not sure if I should answer this as its a few months old but seeing that alot of people look here shows they are interestd. So heres what I know.
Strata can be thought of as a character catagories for npcs. Depending on class, race or importance to the game. Now hit strata is usually used in discipline effects on each type of strata or class of npc and is just an extention of the Hit Human\Hit Supernatural effect types.
Strata 1 is comprised of; Sabbat vampires, bloodhunt vamp, tzim creations, blood guardian, the sheriffs minions and mings tenticles.
Strata 6 belongs to; Bach, the Sheriff (manbat form) and Ming herself.
You wouldn't want "Blood Boil" to have the same effect on Ming as a normal Sabbat vamp, so you can specify to not allow it (Blood Boil) to kill the npcs in Hit Strata 6.
Just to make it worth it..I'll list them all for any future modder.
Strata 1= Sabbat vampires, bloodhunt vamp, tzim creations, blood guardian, the sheriffs minions and mings tenticles.
Strata 2= Sabbat vamps, Bloodhunt vamps, Tzim creation
Strata 3= Warehouse thugs, Sabbat vamps, Chasity, Julius
Strata 4= Lasombra vamps, Tzim creation, Mings Proxy (minor boss), Ventrue ghouls
Strata 5= Sabbot Mage, Killer, Asian vamp, Hengeyoka, Yukie, Gargoyle, Sabbat Leader, Sherrif, Andreil, Pisha,
Cont.....= Jezabel, Chang Bros, Kanker and Burno (seems to be all boss types)
Strata 6= Bach, the Sheriff (manbat form) and Ming herself.
Strata 7= Nos Attacked (guess this is the nossy that gets ambused), Vamp Critical and the werewolf