When you select a deployment area it must be at the same stage that you are promoting to. For example, if you are promoting to the SIT stage, any area that you select must be attached to that stage. You cannot select an area that is not at the same stage.
If the deployment area list is empty and no deployment operation has been requested, no deployment areas will be updated on the target stage during promotion.
If you have the appropriate privilege on an area you can choose which default deployment areas of the target (promote to) stage are populated.
If you by-pass stages during a promotion, default deployment areas assigned to the by-passed stages are not deployed to. For example, there are default deployment areas attached to the SIT and QA stages. You promote from the DEV stage straight to QA by-passing SIT. The QA default deployment areas are populated, however, the SIT default deployment areas are not deployed to.
You do not require any privilege to deploy to default deployment areas as this privilege is implicit (permission is given automatically if you have the privilege to promote to the stage).
You do require a privilege to deploy to non-default deployment areas as this privilege is explicit. Because the area is not a default area you must be granted the privilege on that area before you can deploy to it.