In Selective Repeat, Sender and Receiver both have a window size of n. But to avoid sending a duplicate packet, or sending the wrong packet that has the right number, Sender and Receiver use different sequencing.
Given, n = 32.
So, Sender numbers from 0 to 31.
Receiver numbers from 32 to 63.
What sender sees as 0th frame, receiver sees as 32nd.
What sender sees as 1st frame, receiver sees as 33rd...
One might say that receiver sees the frame numbers in "Excess 32"
So, When sender sends 100th Frame, it got sent with number 99 (because counting starts from 0). Receiver saw it as 131. (Let's not perform modulo right now for simplicity)
So, now receiver expects 132nd frame. => 132 mod 32 => 4th frame. But hold on... Receiver sees it as "Excess 32", remember? This means the receiver expects 4 + 32 = 36th frame.
As per some authors, in SR, Receiver won't send the ACK of expected frame, rather it'll send the ACK of the frame successfully received.
So, it'll send ACK of 131th frame. So, 131 mod 32 => 3rd frame. Do Excess 32 => 35th frame.