This is the 9-years-bugging-me thing from a couple days ago.
I have used doors and windows of my own making since Archicad 8. When I started, it seemed essential to getting our projects looking the way we want; the Archicad Library items just didn't do the job. I know they have improved over the years, but I don't know how much, because I don't use them. I'm sure they are better in many ways than mine, being more full-service, but that's the thing: Mine do exactly what I want, and they don't do other things. When I need a feature, rather than wishing for it, I add it. This is the still-underrated power of GDL.
I'm not saying you should develop your own doors and windows, you likely shouldn't. But having started, I continue. My libraries work well, and I don't pay much attention to changes in that part of the Archicad library. But in AC17, Graphisoft made quite a big change in the behavior of the opening tools themselves, which exposed what was previously an inconsequential limitation in my doors and windows.
One of the things I decided very early on to ignore is the "Flip" button in door and window settings. If you're using my library parts, you don't use this switch. You rotate and/or mirror the element. In our practice, which is US residential mostly wood framing, the flip switch doesn't offer any advantage.
I always sort of wondered why this was, but it was never a priority because I don't use the feature and don't feel compelled to start.
But in AC17, the new UI of placing openings essentially allows you to place the opening flipped, which leads to a confusing user experience if the library parts don't support flipping. So I finally took the few minutes required to get to the bottom of it. (Search for "flip" in GDL manual, Google "WIDO_FRAME_THICKNESS", read this Archicadwiki article, at least that part of it.)
Simply put, when the flip switch is activated, the door or window is mirrored across the "reveal side" face, and shifted by the "nominal frame width", which is a special value in the compatibility options of the details tab. This value can be a length or a GDL expression that uses parameters. It is zero by default, so the shift that should follow the mirroring doesn't take place. I don't use the built in "reveals" (they are also on the ignored feature list), so for my doors and windows this value is just the wall thickness (global variable WALL_THICKNESS, though I still prefer the old name C_).
I'm still not flipping my doors and windows, but now they don't look ridiculous when placing them in AC17.
There is one case where this info might matter to users that doesn't have their own libraries: When saving a custom, slabified opening from the 3D window. When you place that part in AC17, you will get the confusing flipping behavior unless you set the nominal frame thickness to WALL_THICKNESS, or to the dimension of an actual frame component, if appropriate. It just shouldn't be zero. It would be nice if the saving process for those parts did this automatically.
I also don't like that they use a sun icon to represent the reveal side as if the reveal side were always outside, but that is another topic.