cloak of the bat and concealment, What does it mean?!

[Vadenor]

Ok, so there is an interesting item in game called cloak of the bat

+2 ac
Immunity darkness
Cast darkness 3/day

Does this mean, you are immune to the darkness spell as in

enemies do not get a concealment bonus if you attack them?

You do not get a concealment bonus if enemies attack you?

concealment bonus is canceled and attack each other normally?

And will this concealment canceling effect affect other concealment spells such as ghostly visage?

Or is concealment just horribly buggy and no one knows that the heck will happen?

thank you in advance.