The formula used for deciding required and optional fields should be as follows: 

• Ask for as little information as needed.
• Make required fields only those that you absolutely need to proceed to the next step or fulfil an order.

 With a recent site reviewed, I noticed that the following information was set as required to complete a checkout process: 

• phone number
• how did you hear about us?

A phone number may be nice to follow up if there is a need to clarify order details or the address. However, this is also likely to cause reduced orders because many customers do not like giving out their phone number.

Asking your dream customers how they heard about you could also be very helpful in identifying best sources of new potential customers. However, the issue here is again privacy plus you should be able to extract this information from your website analytics. And if you have your website analytics tied with your CRM software with an appropriate custom integration, you may even learn what keywords your customers used when purchasing a product vs. customers just telling you they found you using a search engine.

Ogy Nikolic
Joomla UX Designer