This syntax was introduced to simplify the addition of future conditions. It basically changes the "sense" of the check from true to false. In other words, the statement If,Not,%var1%,EQUAL,%var2%,command is equivalent to If,%var1%,NOTEQUAL,%var2%,command .
If,Not,<Condition>,<Command>
If,Not,%var1%,EQUAL,%var2%,FileCreateBlank,C:\myFile.txt