AS PER MY OBSERVATION, THE FIRST THREE ALPHABETS ARE THE SERIES USED BY THE DEPARTMENT TO ALLOT THE PAN NUMBERS.
INITIALLY ALL THE NUMBERS WERE ALLOTTED FROM "AAA" TO "AZZ" SERIES, THEN ALLOTTED FROM "BAA" TO "BZZ" SERIES AND NOW THE PAN NUMBERS ARE BEING ALLOTTED IN THE SERIES "CAA" TO "CZZ" AND IT MAY BE CONTINUEED LIKE THAT UPTO "ZAA" TO "ZZZ"
First three number shows the area code of income tax department 4 rth shows firm or individuals 5fth alph of first name of surname others are sr. number