First there is no documentation. If there is a cs file with the same name than the mission the Engine calls the .net csc and execute the file.
To get access to the "CloDo programming interface" add references to the assemblies "antlr.runtime.dll", "core.dll", "gamePages.dll", "gamePlay.dll", "gameWorld.dll", "HostView.dll", "maddox.dll", "part.dll", "sound.dll" and "Strategy.dll" in ..\Steam\SteamApps\common\il-2 sturmovik cliffs of dover\parts\core
and
to the files "partBob.dll" and "Campaign.dll" in ..\Steam\SteamApps\common\il-2 sturmovik cliffs of dover\parts\bob
Then you can examine the classes with the object explorer.
The importent classes for missions are AMission and IGamePlay (GamePlay Object).
There is many try and error expirience, some of the members not working proberly (or not fully implementet at time) or they work in single player and player hosted missions but not on Dedi Server.
There is no event for crossing Front but you can use
GamePlay.gpFrontExist() // true if there is a front line
GamePlay.gpFrontDistance(int army, double x, double y) // double distance to front line < 0 if army == player.Army() > 0 if oposite
And with the OnTickGame() - Event (one Tick is 1/30sek) you can check by your own if a plane cross the line