"Sales" gets the business.
"Fulfillment" produces the product / service.
"Administration" provides the infrastructure that allows the other two activities to function.
They are equally important, co-dependent activities so the relationship between them should be synergistic with clear, open communications.
In a perfect world the three functions should meet often to discuss future schedules, desired changes in offerings, changes in procedures, and methods of constantly improving.
In large companies large departments perform the three activities.
In micro-companies, a handful of people (often just one person) may fulfill all functions.
In either case, the division of labor is determined by dividing assets (time / investment / focus) among the needs of the company, its people, and its clients.
I like to describe these activities of a company as being like a child's teeter-totter.
Think of "sales" on one side, "fulfillment" on the other, and "administration" as the balance-point or fulcrum.
If the goal is to keep the company utilizing assets to the maximum, "administration" continually moves the balance-point of investment to the side that has the greatest need at the moment.
If the "sales" side is very effective, "fulfillment" has to be increased or the business will not be able to produce the quantity the "sales" side created.
If there is greater production capacity than orders, more emphasis must be placed on activities that get the product sold or the revenue stream (and those who produce that stream) will suffer.
It would be easy to keep the teeter-totter balanced if we were in a constant marketplace where supply and demand were constant but the marketplace is forever changing so "administration" must be constantly looking forward, analyzing sales cycles, production cycles, threats of competition, the velocity of changing tastes and fashions, opportunities, and technological changes to adjust that balance point.
Every so often kids unbalance their teeter-totter and someone gets hurt.
Don't let that happen to your business.
Keep it all in balance.