I've been flying EWR-ORD on UA for over a month as a Gold

CO Silver = Star Alliance Silver.

As a *A Silver, you will be able to use the premium check-in line and premium security lines. For boarding, you will not be able to use the Red Carpet (nor will any CO Silver, Gold, or Plat). You will be able to board via the regular line before UA's standard boarding groups.

Based on the GA, *A Gold may be boarded 1st, or *A Gold and Silver will be invited to board at the same time.

Hope this helps.