Natalie,
I believe the 'all possible models' option in the stepwise platform simply considers all combinations of factors up to a set number of factors being in the model. That is, if you have 3 x's, call them A, B and C then JMP would fit all the following models:
A
B
C
A+B
A+C
B+C
A+B+C
For each of the models it would calculate fit and then list them so you could identify the best. So, instead of using a logic based stepwise system, it's a 'brute force' type of model fitting.