What does "External Checks" involve for citizenshi

[johnty]

Hello
I see from the pinned thread that it take about a month, the citizenship appliations are decided. In my case it has been over 4 months. I called UKBA and been told that they are waiting for external checks.
The documents I sent with appliation are
Passport
council tax bills
hmrc documents

What do they mean by external check?

are they confirming the above documents I sent or something else?
I have nothing worry about. I am clean by the way! just curious..

regards