SSO Issue: new login url on sandbox refresh

We have an SSO which we've been able to reconfigure any time we refreshed a sandbox, but this latest sandbox refresh added something to the Login URL which didn't previously exist:

• Login URL before refresh: https://xxx--sandbox.my.salesforce.com?so=OrgIDNumbersLettersHere
• Login URL after refresh: https://xxx--sandbox.my.salesforce.com?so=OrgIDNumbersLettersHere&sc=randomLettersAndNumbersHere

Anyone have issues with this? Anyone know what the SC stands for in the new URL?