UKBA reference number of AN form

Hi,

What is the UKBA reference number? Is it the number on the residence permit stamped on my passport which is written in red and starts with UK and then seven digits? Other than that I have never had any home office letter only once just to transfer my ILE (indefinite leave to enter) to new passport and they made it permanent residence. They only gave me a 'case ID' in that letter and no reference number.

Thanks