Finding Your Salesforce Org ID
Your Salesforce Org ID uniquely identifies your org. Two customers can share a company name; they can't share an Org ID.
Where to find it
Configuration → Help & Troubleshooting → Step 4 shows it directly.
It's also in Salesforce Setup under Company Information, as an 18-character value starting 00D.
Why it's asked for
Without it, support is matching your case against a company name, which is ambiguous across sandboxes, production and multiple orgs. With it, your case routes straight to the right org first time.
The support case form includes an optional Salesforce Org ID field. Filling it in avoids the first round-trip.
Sandbox or production
Give the ID of the org where the problem happens. A sandbox has a different Org ID from the production org it was copied from — sending the production one for a sandbox issue points support at the wrong place.
Xero
Real Estate
Equifax