Change set deployment connections and inbound and outbound flow
Key Takeaway
The target org is in control. Before a sandbox can upload a change set to production, an administrator in production must allow inbound changes from that sandbox. After upload, the change set has no effect until someone in the target org deploys it from the "Inbound Change Sets" page.
A change set moves customizations from one Salesforce org to another, such as from a tested sandbox into production. You build an outbound change set in the org you are logged into and upload it to another org. For the org that receives it, the change set is an inbound change set. Before anything can move, the two orgs need a deployment connection. The target org must also authorize uploads from the sending org.
Outbound and inbound change sets
A change set carries metadata, not data. You can include only changes you can make through the "Setup" menu. So you can't use a change set to upload records, such as a list of contacts.
The direction depends on the org you are logged into. An outbound change set is one you create in your current org and send to another org. A common use is sending customizations that were built and tested in a sandbox to a production org. An inbound change set is one that another org sent to the org you are logged into.