Saves your map as a .mir3 map file, whatever that is.
Never used it.
It picked my interest and I did some investigating. First the theoretical findings.
Invert means "
to turn something upside down or change the order of two things". The clue seems to mean most likely swapping two layers and primary suspects are front and middle layers, given the mir3's way more extensive use of middle layer compared to mir2, which only uses middle layer for ground patches, various ground features acting as an alternative back layer.
In practical investigation I opened the first map in mir3 client (from 2008), the '0.map' and saved it as '0.Mir3' map. Then tried to open it in the editor and found it opened just fine. Showing front and middle tags in turn and taking screens, I found that indeed, the front and middle layers are swapped on the '0.Mir3' map compared to the default '0.map'
Mind you, the mixing of front and middle layers on mir3 maps is quite atrocious, nothing new here as we settled on that a while back. Like a tree which is built mostly on the middle layer has several image strips of it on front layer. Reason I mention it is that, curiously enough, the image strips can apparently be on the middle layer in mir3 and yet show up properly aligned height wise in game (didn't test but I presume it would be OK), as well as in the editor.
In mir2 (C# anyway), we know it doesn't work that way. Image strips placed on middle layer that look properly placed in the editor are shifted down when viewed in game. Apparently mir3 engine must display them correctly in game as well as in the editor. Go figure.
I tested inverting mir2 map and it does some partial layer swapping and in game the map has shifted image strips of those front layer strips that were swapped to the middle layer, as expected. In the editor, it looks OK as always.
The function doesn't have any practical use I can imagine, perhaps if you were strictly consistent with what you put on what layer but even then. Would make sense if you could select an object that you placed by mistake on middle layer and save it with inverted layer function button on front one. Will test that later today.
Now with horror I noticed the second grasp function, the mir3 one, that I use quite often, includes the word 'invert' in it. Too tired to investigate that right now, I didn't notice it would be swapping layers, just includes middle object with front ones compared to the grasp of mir2, which copies only the front layer selection.