The Domain Access module is a node_access() module. For additional developer information, see http://api.drupal.org/api/group/node_access.
By design, the module sets access to content based on the current domain that a user is viewing. If a user is at one.example.com, they can see content that is assigned to that domain or to all domains.
Assigning Domain Access
Users who have the 'Set domain access status for all content' permission can assign any node to any or all registered sites. During node editing, a series of options will be displayed as checkboxes or a multiple select list under the heading "Domain access options":
Publishing options:
[] Send to all affiliates
Select if this content can be shown to all affiliates. This setting will
override the options below.
Publish to: * (required)
[] Drupal
[] One site
[] Two site
Select which affiliates can access this content.
If you select 'Send to all affiliates,' the node will be viewable on all domains for your site. Even if you select this option, you must select at least one domain for the node.
When creating new content, the currently active domain will be selected for you by default. You can change this behaviour with the Content Defaults feature in the Advanced Module Configuration.