The answer is communication. Milady states this plainly as a figure caption in its teamwork section: communication is the key to teamwork.
The surrounding passage explains why — working on team-building characteristics builds a strong sense of loyalty to your shop, and clients themselves respond to a barbershop that clearly functions as a team. Determination, competition, and ambition are all individual traits; communication is the one the book ties specifically to teamwork.
Milady Standard Barbering, 6th ed., Ch. 20 (Working Behind the Chair), p. 737: "Communication is the key to teamwork."
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