enum abstract NullSafetyMode(String)

Loose safety. If an expression is checked != null, then it's considered safe even if it could be modified after the check. E.g.

function example(o:{field:Null<String>}) {
	if(o.field != null) {
		mutate(o);
		var notNullable:String = o.field; //no error
	}
}

function mutate(o:{field:Null<String>}) {
	o.field = null;
}

Disable null safety.

Full scale null safety. If a field is checked != null it stays safe until a call is made or any field of any object is reassigned, because that could potentially alter an object of the checked field. E.g.

function example(o:{field:Null<String>}, b:{o:{field:Null<String>}}) {
	if(o.field != null) {
		var notNullable:String = o.field; //no error
		someCall();
		var notNullable:String = o.field; // Error!
	}
	if(o.field != null) {
		var notNullable:String = o.field; //no error
		b.o = {field:null};
		var notNullable:String = o.field; // Error!
	}
}

Full scale null safety for a multi-threaded environment. With this mode checking a field != null does not make it safe, because it could be changed from another thread at the same time or immediately after the check. The only nullable thing could be safe are local variables.