Team's Response

We are unsure of what she/he means by "construct it another way" and "kind like for accident happen after step 1".

Image below for what we thought extensions meant and the reason why we wrote it this way:

Moreover, what we did is similar to the 3b in the notes:

Items for the Tester to Verify

Team chose [response.Rejected]

Reason for disagreement: Sorry for the ambiguous phrases I used during PE (due to time constraint).

In my opinion, the 3b in the notes (the example you provide) is correct, there are 2 reasons:

First, it is an alternative flow, as "future transfer" implies that the OBS won't transfer money immediately (so it is different from MSS)
Second, based on the rule of numbering, 3b is happen just after step 3 of the MSS "user enters the requested details" (user still need to enter details like account number)

I feel that the UC3 in your DG is a bit different from the example in the notes:

First, compared as an alternative flow, 1a is more like another options user can choose in step 1, both 1 and 1a have the same flow after them.
Second, since it is like a different option in step 1, it is problematic in numbering because it does not happen "just after step 1". At the same time, you also cannot label another number for this extension.
Another problem: didn't mention "find a student" is UC5

Can construct in this way: changing step 1 to

(similar to step 6 in the notes here)