The fundamental difference between PTO and ATO items is the way they are ordered. Both are composite items but for PTO there are no choices while in ATO there are.
PTO - Pick To Order is a fixed configuration of a product which you need to order. No changes can be done in the configuration.
ATO - Assemble To Order products have options from which the user can configure there final product.
For example consider the buying of a computer. Now there are various sizes of monitor that can be available. Say X is the entire computer. If its a PTO then the monitor given shall be the one that will be delivered. But if its an ATO then the user can select the monitor that is a 15 or a 17 .
This ismade possible in ATO items through the presence of Option Classes.