In the world of Dynamics 365, configuration/customization/extension does not equal the old world assumptions of "custom software"
When thinking of the experiences or backgrounds needed for any specific project, don't let assumptions trip you up. These terms can help with your communications.
Configuration – knowing of and Answering all the questions related to setting up the system for a specific customer. These include all the options under SETTINGs including but not limited to items such as turning on auditing, configuring number formats, turning on relevance search, setting up duplicate detection, configuring territories, setting fiscal year end and more. This is done on every Dynamics 365 project (hopefully) and there are more than 50 questions.
Customization – A functional consultants power to configure the system for specific business models such as renaming entities, adding entities, adding system charts, lists/views, adding workflows, creating business rules, formatting the data entry fields and forms, creating PowerApps, setting up relationships between entities and more. The term customization often waffles between configuration and extensions depending on who is using the term.
The world of technology is deep and wonderful.