Do you want to be functional? I don't. Functional training has more than one meaning, at least for me. In the health and fitness industry one could surmise that training for functionality relates to your body being able to perform it's daily activities with more efficiency.
I assume, because I think it's a vague message being delivered by the indusrty, meaning fake trainers, gurus, etc. that functional training also applies to athletes, but because of my background in sports specific training I've yet to see a coherent account of that.
On a personal level, as a result of being paralyzed seven years ago, now fully recovered, having a physical therapist tell me their goal was to get me to be functional, made my blood run ice cold. Functional my ass was my only thought, along with my decision seven years ago to never go back again to that physical therapist.
Functionality in this example obviously had a very different meaning for me verses the physical therapist. My opinion, and approach to being functional is to be specific in the way you achieve functionality. If you're a legit trainer you'll take it a step further with whom ever you train.
Identify the functionality of what your client would like to achieve, and make them understand to be functional is fine, but it's mostly aspiring to be average. Then you recommend and construct a step by step process with a specific training program addressing the body parts in the order in which they should be trained.
You backup your training strategy by telling them the how and why of your proposed training methodology. In essence, your goal for them is to reach their highest level of functionality which can only be accomplished with a direct approach. At least, that's what I'd do.