what is the nickname for the lower biological shield?

hi guys, i have a question for an essay i’m writing, i know Elena is the upper biological shield from this subreddit and some of Alexander Kupnyi’s videos, but i am unaware of the lesser known lower biological shield below the reactor’s nickname, if it even has one.

It got pushed down 3-4 meters and in room 305/2 you can enter the shaft, otherwise i know little info about it.