Cal. Code Regs. tit. 4 § 1896
A trainer who has in his care a substantial number of horses, or who is actively participating in more than one race meeting at one time, may employ an assistant trainer, who shall be equally responsible with the employing trainer for the condition of the horses in their care. The name of the assistant trainer shall be shown on the official program along with that of the employing trainer. If any licensed trainer is prevented from performing his duties by illness or other good cause, and is absent from the track where he is participating, the stewards shall be immediately notified, and at the same time, a substitute trainer, acceptable to the stewards, shall be appointed and such substitute trainer's name shall be shown on the official program if possible. The stewards shall be advised immediately when the regular trainer resumes his duties. A substitute trainer shall be responsible with the regular trainer for the condition of the horses in his care. Assistant and substitute trainers must be licensed trainers.
Cal. Code Regs. Tit. 4, § 1896