Sandbox org identity and user considerations
Key Takeaway
A sandbox is a separate org with its own org ID, and the copy changes each user's username and email address.
A sandbox is a separate org that Salesforce copies from a source org, so teams can do development and user acceptance testing (UAT) apart from production. The source is production for new or refreshed sandboxes and another sandbox for clones. The new org ID and the changed usernames and email addresses keep the sandbox separate from production and stop it from emailing real users. These changes also explain many common sandbox problems, such as broken hard-coded IDs, failed logins and frozen users.
Sandbox org IDs
Every sandbox is its own org, with its own org ID, separate from production. Salesforce creates a new org on each create and refresh request, so a sandbox's org ID changes every time it's refreshed.
Where a production or sandbox org ID is used in text values and metadata, Salesforce inserts the new sandbox org ID value. A script or process that depends on a hard-coded org ID, such as a test script or Web-to-Lead, must use the sandbox's current ID. When you deploy the changes to production, you must update it with the production org ID.
To find the ID of the org you're logged in to, enter "Company Information" in the Quick Find box in Setup, then select Company Information.