Actually having that discussion with Nr_RaVeN right now. Basically the editor does a bunch of programmatic checks only. There are things you can do that will pass the editor but will trigger issues from a mechanical stand point once on the server or from a play ability stand point.
An example of a mechanical is that you can actually go and create a forest by individually placing every single tree. Its tedious but can result in some stunning results and actually allow you to the tree line more close to the shoreline. However, doing this has caused some issues in the past such as killing players frame rates and from what I was told put an extra strain on the server.
Another instance was we had a case where some custom field work was done but it somehow impacted the AA guns on the field. The editor was fine with everything but when I actually went spawned in the gun and fired off a few rounds the tracers were actually offset to the right of where they should have been. So for example if anything is done to an airfield it is part of my testing routine to actually go and spawn in the guns, spawn in the hangars, and at the spawn point to make sure everything is okay. Had another case where work was done that impacted the ability of LVTs to transition from water to land. Also have many instances of bridges levitating in the air. Not good for bridges but opens up an interesting possibly of making barage ballons for WW1 terrain or WW2 so acceptable for a barrage balloon but not for a bridge. All of these type of issues have happened in the past and the editor checks didn't catch them, only manual testing caught them. Which is why HTC really doesn't allow for much customization for MA terrains.
From a play ability stand point. If you know what you are doing you can actually take an existing plane and replace its 3D model (ask MrMidi since he is the 3D modeler) converting it into a new plane. You could say take a P51 and replace it with a He 111. It would still have the flight characteristics, damage model, etc. of the P51 but to the players it would actually look like an He 111. While doable, no you would not want this because a player comes in and thinks they are selecting a P51 and then get something completely different. You could replace the CVs in a terrain with Akagi CV. Now this is acceptable but would have to be tested to make sure that landing and take off still works correctly and manning the guns still work correctly.
Basically when adding new things to terrains you always want to add and not subtract. For instance you don't want to make re-arm pads destructible by players even if the editor allows you. Also you don't want to put Task Groups to close to land because it can cause server issues.
So its basically a whole host of what are probably termed best practices. If you create a 3D object it needs to be inspect to make sure that while the editor accepts it that it won't actually cause issues. Technically with the editor and 3D objects you could model the City of London in exact detail. The editors checks would allow you to do this. However, doing could crash the terrain since you are modelling every building individually and the strain on the server might be to much. As I said MrMidi is better one to chat about the exact details of modelling but I know Dale has also talked about the issue of how much detail can cause server issues (the whole do you really need to model the rail for the stairs of the field tower in exact detail discussion).
The more you customize things with new objects or modified objects the more things manually have to be tested and reviewed.
Hope that answers some of your questions.
