A fiduciary holds money in a position of trust for others. Think of a valet holding your car keys: the keys are in his hand, but the car is not his.
Premium funds (Ins. Code §1733): all funds a licensee receives as premium or return premium are held in a fiduciary capacity. Diverting them to personal use is theft.
A return premium received to pass on to a client is fiduciary money.
Financed premium counts only once the licensee actually receives it; premium a finance company has merely agreed to advance does not.
Earned commissions and rent paid to the agency are the licensee's own money.
Premium trust account (Ins. Code §1734): a licensee may remit premiums, less commissions, to the insurer, or hold them in a separate trust account insured by the FDIC at a bank or savings and loan in any state, or use the method in §1734.5. The account must hold at least the net amount owed.
What may be mixed in: the licensee may commingle funds the licensee deems prudent to advance premiums, to set up reserves for return commissions, or to meet contingencies in handling premiums. Paying office rent from the trust account is not one of those purposes.