I saw this a few days ago. I have a Sabre subscription and can see many buckets. Here is an example:

DL/** 4006 SAT MSP 18 JUN J1 C1 D1 I1 Z1 W0 S1 Y0 B0 M0 H0 Q0 K0 L0 U0 T0 X0 V0 E0

Why is S=1 while W=0? Well, there is one unassigned C+ seat on this plane but since the flight is sold out in Y (Y=0) it is not bookable. If you try to book this flight it is sold out in Y and C+. The other day I saw S8 with Y0. My kids were booked on that flight and when I pulled up the seat map of their reservation there were 8 unassigned C+ seats. Of note, there were two C+ seats held back for airport assignment that were not captured in the S total.

I do not believe you can book a ticket in S. Also, I believe this is upgrade code to C+ for Medallions booked in Y.

Hope this helps. I believe this is correct. But an open for further insights.