Autofill won't work with Filter.
Assuming you want to assign seatnumbers whre Col A has English... (with
first row having headers) enter this formula in row 2 of any col and copy down
=IF(A2="English",COUNTIF($A$2:A2,"English"),"")
This will enter 1,2, ... to those rows having English.
You can format them as 000 to get 001,002,...
"alibaba" wrote:

Seat arrangement of students in Exam
Let me make it simpler.
column A stores the exam subjects that I am going to filter. I found that
after applying the data--filter--autofilter(with some creteria) on column
A, a certain subject(say English) is filtered. Then I try to use autofill in
column B to assign seat number to the students(001, 002, 003,..). However, I
cannot fill in the series. Can anybody help me?