In this example a Auto-Increment ID is generated in the format AA001 AA999 then rolls to AB001, etc. with a maximum value of ZZ999

Here are the formulas used:

Starting in the first row:
3 Digit Number:
=IF([Number Increment]@row = 0, 0, IF(MOD([Number Increment]@row, [Maximum 3 digit value]$1) = 0, [Maximum 3 digit value]$1, MOD([Number Increment]@row, [Maximum 3 digit value]$1)))

ID
=CHAR([First Letter]@row) + CHAR([Second Letter]@row) + IF(LEN([3 Digit Number]@row) = 1, “00” + [3 Digit Number]@row, IF(LEN([3 Digit Number]@row) = 2, “0” + [3 Digit Number]@row, IF(LEN([3 Digit Number]@row) = 3, [3 Digit Number]@row)))

Starting in the 2nd row:
First Letter:
=IF(AND([Second Letter]1 = 90, [3 Digit Number]1 = [Maximum 3 digit value]$1), [First Letter]1 + 1, [First Letter]1)

Second Letter:
=IF(AND([Second Letter]1 = 90, [3 Digit Number]1 = [Maximum 3 digit value]$1), 65, IF([3 Digit Number]1 = [Maximum 3 digit value]$1, [Second Letter]1 + 1, [Second Letter]1))

Number Increment:
=[Number Increment]1 + 1