Your answer is in your question!
"Freestyle" means exactly that.

The hair isn't knotted (vented) the the base to go or lay in any particular direction.
Therefore you can comb & style the hair in any way you want.
Forward, back, spiky, up, flat etc.

People choose to get their systems vented in a particular direction (with custom orders) once they have a definite style & direction they prefer their hair to always go in.
You can order a system that has different direction venting in different areas.
eg. The top combed forward from the crown, sides swept back and spiky front.

Some people always have a parting or break or they have the crown on the left or right hand side rather than central.

But you could also achieve that same look with a freestyle ventilated system.
A freestyle system is usually a good starting point for first time wearers as it gives them options whilst they are getting used to wearing and how they might want to style their hair.

Stock systems are often freestyle because they will appeal to more people.

I hope this helps.