It's a tube that projects up inside the filter that maintains a certain depth of oil inside the filter after engine shutdown. (filter is mounted holes down)
The adbv will prevent oil from draining out the inlet holes (as long as the adbv is working anyway), the tube keeps the oil from draining out the outlet hole (maintains the level at the height of the tube anyway).
My Nissan truck has one built-in to the filter mount at the block.
It sortof "extends" the outlet hole up inside the filter. It would also help seperate any potential sediment away from that outlet hole as well.
