What are Anchor and Passthrough used for in popover segues?
These are properties of the UIPopoverController, not the segue. Thats why if you look at the documentation for the UIStoryboardPopoverSegue, you won't find these two properties.
The anchor is telling the UIPopoverController where to point its arrow.
The passthrough is an array of views that the user can interact with while the UIPopoverController is displayed.
The documentation is here: https://developer.apple.com/documentation/uikit/uipopovercontroller
and the section going over the passthrough views is here:https://developer.apple.com/documentation/uikit/uipopovercontroller/1624654-passthroughviews
UIPopoverController seems to have been deprecated recently. However, I still see Anchor and Passthrough fields in the storyboard segue inspector.