A project running on a particular gateway, can access devices on multiple gateways, or OPC-UA servers.
I my case, the main Ignition server has no local devices connnected. There is also a copy of Panel Edition running on another box and it has the all the devices connected to it. There is also a motion controller by Bosch Rexroth that has an OPC-UA server built in.
When in Designer working on a project on the main Ignition server, when you browse OPC you see all off the OPC-UA servers and their devices and tags.
I can see the Panel Edition and all of the devices connected to it.
I can see the main Ignition server that has no devices attached.
I can see the motion controller and all of the tags declared and published therein.
This is really sweet.
I hope this helps.
Dennis