It is obvious I have not explained myself clearly. Perhaps my training as a fitter has skewed my vision and approach or the way I explain things. When we do a pro level fit, we set up the size cycle to a specific frame size based on body measurements. Top tube first, seat tube second. From that we make adjustments to the movable parts, ie, saddle, eat post, stem, bars, pedals, cranks with input from the rider and solid measurements (such as cleat position and bend in knee). The frame size is not in question, only the "fit" on that particular frame is. Experienced bike shop personnel know the fitting "key points" are saddle height, hip position, and cleat placement for lower body, and shoulder position, head tilt, and arm extension for the upper body. There are parameters to work within to allow adjustments up or down in the future based on intended use.
All of this is based on a question asked in the pre-fit interview or in the initial conversation with the customer if no fit is involved: How do you intend to use the bike?
To leave the decision up to the customer is not professional unless accompanied by an explanation of why you cannot specify a specific frame size. In my opinion if you really don't know what size would work best, that should be a warning sign to someone looking to an expert for guidance. I wouldn't accept if from my tailor, and I wouldn't accept if from a bike shop.