Suppose that I wish to adopt either SAP S4/HANA or ECC6 ERP for my organization which operates in the manufacturing domain.
I want my customers to place an order and I wish to give them the ability to customize it as well.
What are the features available at both S4/HANA and ECC6 that would allow me to customize orders and specify product design (ie. spec, size, etc)?
I assume, that you are either a business user or an IT department member, with no prior SAP experience, or this is a homework question
You can start by searching for 'variant configuration', I can recommend the Best Practices content on the topic, especially the *.ppt for the scenario overview and the business process documentation as a starting point.
I also read your other question - from what I can see, your best option is to contact a SAP implementation partner and, most likely also a business consulting company. Project scope definition is too important to rely on random people in an internet forum to provide advice, based on limited data.