So you want the discount to apply only if the Subtotal is less than 100? you need to include this sort of detail in your questions for me to be able to give accurate answers.
your rule:
"SubTotal of items < 100" or "total number of items > 0"
doesn't make sense. using OR "total number of items > 0" will cause the discount to apply whether the subtotal is under 100 or not.
it should be:
"SubTotal of items < 100"
AND
"total number of items > 0"
AND
Custom Condition: isset($_SESSION["PromoCode"]) && in_array("12345",$_SESSION["PromoCode"])