scripting using external assembly

I see this statement the Biztalk 2006 do***entation - "The code in the 
external assembly must be thread-safe. Under stress conditions, multiple 
instances of a map may be running concurrently. "

What constitutes thread-safe? 
Is it enough to make sure all work variables are declared internal to the 
method? 

public class MapperHelper
	{

                    private string var1; // unsafe to assign to this and 
return it ?

		public MapperHelper()
		{
		}

                               public string method1() 
                               {
                                    string var2; // safe to assign to this

and return it?

                                }
	}


Any help appreciated,
Gary Pardue