Deployment connections for change sets
Key Takeaway
A deployment connection alone does not let change sets move. Before a change set can be uploaded, an admin in the receiving org must select "Allow Inbound Changes" for the sending org.
A deployment connection is the link between two related Salesforce orgs that lets one org send change sets to the other. Connections exist only among a production org and its sandboxes. Each receiving org must also authorize uploads from each sending org, so the receiving org decides which orgs can change it.
Which orgs have deployment connections
To send a change set from one org to another, you need a deployment connection between them. You build an outbound change set in the sending org. The receiving org sees the same change set as an inbound change set.
Deployment connections exist only among orgs affiliated with a production org, which means the production org and its sandboxes. You can't create a connection between arbitrary orgs.
The "Deployment Connections" list is populated automatically with your production org and all its sandboxes. You can deploy between any of these orgs, but not to any other org.
Priya is the admin for a production org with three sandboxes named Build, QA, and Training. When she opens the Deployment Connections list, it already contains her production org and all three sandboxes. A connection exists between production and each sandbox. A connection also exists between each pair of sandboxes, such as Build and QA. Priya also has a separate practice org that isn't affiliated with her production org. That practice org has no deployment connection to her other orgs, so she can't deploy change sets to it.