For a Linked List to be created, both parties sharing the list (an Advertiser and a Network) must have an OPTIZMO account. The Linked List is created when a Mailer Access Key (MAK) is provided by an Advertiser to a Network partner, and that MAK plugged in to the proper area during the Networks' list creation workflow. This process "Links" that lists' data between the two accounts without the need for manual downloading and uploading.
How Opt-Out links will work
As a Network user, if an Opt-Out Link is being provided to you via the Advertisers MAK, the Opt-Out Link generated from your system for your Publishers will utilize your system's domains, however the Opt-Outs collected by your unsubscribe link will pass back to the Advertiser.
The Opt-Out Link is actually attached to a sub-id in the Advertiser's account, because that has the source Opt-Out List where Opt-Outs are stored. Your Network account just receives Opt-Out counts and stats.
In short, the Opt-Out Links for Linked Lists are generated based on the Advertiser's settings, using the Network's domains, and all Opt-Outs will pass directly through to the Advertiser's original lists.
*Note: When you link a list, some Opt-Out settings in your account are disregarded and the Advertiser's are used. This is by design, as the Advertiser owns the list, and therefore determines the settings around sharing it. You will not be able to edit the "Handling of Opt-Outs", or change Mailer Access to plain text if it is shared with you as MD5.