ZDoom

I've just remembered that I've seen another use [url=https://forum.zdoom.org/viewtopic.php?f=42&t=67485]quite recently[/url].

I think what they were used for was to make a "floor lower to nearest" call in a script work as intended. The setup was for exploding walls. There would be 3 or 4 sectors right beside each other. All of them had the same tag and they all need to lower to the same target height at the same time to give the impression of a hole opening up in a wall. As you can see from the screenshot:

[imgur]https://imgur.com/rQQ0o0R[/imgur]

They started off as closed sectors (i.e. no gap between floor and ceiling - giving the appearance of a solid wall). However, the sectors all have different ceiling heights. So (at least I'm assuming here) having the floors match their ceiling heights would mean that calling "floor lower to nearest" would mean the middle floor only lowering as far as the ceiling height as the one on the left (which would have had its floor height at that ceiling height value originally).

Of course, seeing as how this is being done by a script, the sectors could also have all been given different tags, set up without using "flipped" or "negative height" sectors and lowered using "floor lower by value" instead.

All of the above is supposition, because I didn't make the map.

Instant motion was made by assigning a negative path to inner variables (for example, moving floor up to neareset floor but there is just one floor and it's lower than current position).
I can think about setting a 3D floor by placing a filpped sector into a normal one but this'll probably require support by node builders. Or making a sector which is both flipped and self-referenced, and use it somehow. Or filling flipped sectors with water on level load. Or filling them with solid but destructible material.
Or maybe tricks with vanilla engine but I can't remember exactly, too.

They were definitely used for tricks in the past, not sure what though - instant moving sectors?

As you say, no need now.

BTW is there something "flipped" sectors are good for? (Except Vavoom 3D floors).
I speak about sectors with negative height. Floor height > Ceiling height.
They are probably not needed in UDMF because everything can be set without such tricks. But before UDMF was invented, was there any use for them?
I can think about setting floor to lower value and ceiling to higher value on level load, flipping the sector back to normal before the game starts, and setting a bit in sector properties. This could allow storing an additional bit for Sector Effect set without changing the map format. But I don't know if something like this was actually done.

Thanks again :)

Yes, those things can be set in a MAPINFO lump which doesn't really care about the map format.

Thanks. I need to pay more attention to UDMF.
BTW can I ask if UDMF supports "zdoom/vanilla gameplay restrictions"? For example, setting "allow jump = 0" if map is designed for vanilla gameplay or "allow jump = 1" if jumps are needed to complete it etc.

Something very much like this already exists. The closest that you will get (and I suspect it's pretty close to what you want) is setting the following flags in UDMF mode:

Walkable middle texture
Projectile shoot-through middle texture

If you are editing in ZDoomHexen mode you can still make walkable middle textures, but you can't set the projectile shoot-through middle texture flag, so only hitscans and not projectiles can pass through.

I forget what is possible in standard Doom mode.

This will not stop attacks (etc) passing though the opaque bits of the texture, but (as the flag name implies) the textures can be walked on at whatever height they are so they are very good for making bridges etc and behave basically like normal Doom blocking lines (perhaps with a midbar texture of some sort on them) except that they only block within their height and not the full height of the line.

Two-sided textures which are "hard" only in non-transparent parts. So it's possible to jump over them without the self-referencing sector trick and to shoot thru them (unless you actually hit the bars, not the space between them).

Maybe they must be in "sector height transfer" linedef group because they'll work for cheap "rail style" 3d bridges.

Or maybe a partial version of this feature (jumping over the texture height but the texture itself does not block bullets at all), "rail bridges" support only.