Inbound change set validation versus deployment
Key Takeaway
Validate is a test deployment that changes nothing, and Deploy commits the changes. Validate doesn't support a field type change between Master-Detail and Lookup. For that change, test with a full deployment to another test sandbox instead.
An inbound change set is a package of metadata changes that another Salesforce org sent to the org you are logged in to. Nothing in it takes effect until you deploy it. Before you deploy, you can validate the change set to find out whether the deployment will work. Validation changes nothing, while deployment commits the changes.
Validate versus Deploy
Validate runs a test deployment of the inbound change set, which shows the success or failure messages you would get from an actual deployment. Validate doesn't deploy any changes.
Deploy applies the change set as a single transaction. If the deployment can't finish for any reason, the whole transaction rolls back. After a deployment succeeds, all its changes are committed. You can't roll back a successful deployment.
You deploy an inbound change set as a whole, not component by component.