That second arg can be whatever frame/object/control you need it to be for anchoring. If you want to anchor your addon's frame to the top left of the screen in general, use GuiRoot.
Using the zgoo addon and looking at the 'zz UI' section... That is all the extant UI controls you can anchor off of... as far as I know.
If there is something on your screen you want to anchor on, it is likely easier to '/zgoo mouse' and use :GetParent() and :GetChildren from whatever is picked up to find exactly what you need.
Using the zgoo addon and looking at the 'zz UI' section... That is all the extant UI controls you can anchor off of... as far as I know.
You can anchor your frame to whatever you want. (So long as it's not circular. - your control anchored to another control which is anchored to your control.....) It doesn't have to be in any sort of list.
If you are just shifting your main frame/window, I would recommend the overloaded API command. It is SetSimpleAnchor. Then you only have to worry about the parent window and the screen coordinates.
SetSimpleAnchor protected-attributes (object anchorTargetControl, number offsetX, number offsetY)
Unless the devs have plans for that function in the future, I wouldn't bet 100% on it staying around for forever. The default UI does not use that function at all, and only uses SetSimpleAnchorParent once.