The description of the HTTP param that's ultimately sent based on that selection is this:
In a redundant gateway environment, set this parameter to 'true' to retrieve local-specific files from the redundant peer over the gateway network. These files will be stored in the .gwbk alongside the local-specific files from this gateway.
Based on that, the intention is to capture things from the "local" resource collection (explicitly hardcoded to be the lowest part of the config inheritance tree; see here for more info) from the redundant peer.
It looks like this is purely informational capacity - sort of by definition you'll be restoring this single .gwbk to the primary and it doesn't appear to (currently?) do anything to send these local overrides to the backup.