A commercial building has a WYE transformer for it's 120vac 1ph power. It has 4 wires going out:
L1 - Hot 120v
L2 - Hot 120v
L3 - Hot 120v
White - Neutral for 120v
Any two of the L1, L2, or L3 lines when hooked to power leads makes 208vac, not 120v. Why? They are 120° separation. Only when they run a 3 phase motor do they kick out 240v.
You do not use the Neutral on a 208 circuit. You would use a second Hot instead.
120/208 "wye" is the current "standard" way of getting 3 phase and 120 volts 1 phase out of a single service, but there are also lot of 120/240 "high-leg delta" 3 phase systems out there which work differently.