Sequence numbers are used to allow for easier editing of your ACL's. The preceding example used numbers 10, 20, and 30 in the ACL lines. If I needed to add another line to this ACL, it would have previously been added after the last linemy line 30. If I needed a line to go closer to the top, I would have had to remove the entire ACL and then reapply it with the lines in the correct order. Now I can enter in a new line with a sequence number, placing it in the correct location.
The sequence-number argument was added in IOS version 12.2(14)S. It was integrated into Cisco IOS Software Release 12.2(15)T.