On the SWAPA site.
1. Click RESOURCES
2. Click SRC tools
3. Click VACANCY PROJECTION
4. Click GET REPORT

Second column from the right shows the approximate seniority number that will be awarded based on what everyone currently has in their vacancy bid.

So for example is BWI says 727X it means that someone from seniority number 7270-7279 will be awarded the bid (if nobody changes their bid before it closes)