Happy to help
It is available for all platforms. OpenFL replicates the whole flash API and uses flash classes when you compile to flash/air, but it has its own classes for other platforms. Platform availability is specified for each class in the api documentation.
Typically anything you can do in as3 can be done exactly the same in openFL, with slight syntax differences.
AS3 Classes that are only available in AIR may require finding an alternate solution, for example here is a function that loads data from a file in a game :
static public function gameLoad(fileName:String):String
var data:String = "";
var file:flash.filesystem.File = flash.filesystem.File.applicationStorageDirectory.resolvePath("save/" + fileName + ".dat");
var fileStream:FileStream = new FileStream();
data = fileStream.readUTFBytes(fileStream.bytesAvailable);
var path:String = System.applicationStorageDirectory + "save\\";
data = File.getContent(path + fileName + ".dat");
I use AIR to compile and test very quickly, but real builds target windows (c++)