What is the definition of a "Simple 2" transaction.

[L. D. James]

I see references to "Simple 2" transactions on many of the tutorial.  
However, I can't find the exact definition of what makes a transaction a 
"Simple 2".  On every reference I see it starts out with something like, 
"Here is another simple 2 transaction."

Thanks in advance for input on this.

-- L. James

-- 
L. D. James
ljames at apollo3.com
www.apollo3.com/~ljames