Usually when a sales representative enters a customer order into the system, an availability check is carried out and a screen that looks like this, pops up.
I hope this helps to clarify at least to some degree. There is so much disconnect between the sales order demand and the production program that a focused effort to align the two is necessary in most SAP installations I have seen. Get started with the decision for MTO and MTS but then look into the availability check and its setup and ability to automate. It's an effort worth taking on.