this is a good question. I don't know how they determine this...

I believe it means that you are the 26th person in line for the month you submitted your application and service center. This number only includes people who registered on this app though so it's not a full accurate picture of your place in line. Also as we have often seen, USCIS does not often process in the order received.

G B .. Thank you

thanks @G B. So, I guess it does not even necessarily mean that it's an accurate reflection of our place in the queue...We submitted May 5, and it shows us at 51/84.