When two auto policies apply to the same loss, the first question is which pays first. California answers it by statute. Under Insurance Code §11580.9(d), the policy that describes or rates the vehicle as an owned automobile is conclusively presumed primary, and any other policy is excess. In short, insurance follows the car: a person who borrows a friend's car looks to the friend's policy first, and the borrower's own policy sits above it as excess.
Two business exceptions in §11580.9(a):
(a)(1) When a business that sells, repairs, services or road-tests vehicles, or its employee, is operating the vehicle, the business's policy is primary. A shop employee road-testing a customer's car is covered first by the shop's policy.
(a)(2) When a vehicle covered by such a business's policy is driven by someone outside the business, such as a customer in a dealer's loaner, the business's policy is excess over the driver's own insurance.
Pro rata sharing: when two policies apply on the same level, for example a car briefly listed on two policies during a change of insurers, a standard pro rata other insurance clause makes each pay the share its limit bears to the total limits. Example: limits of $100,000 and $200,000 and a $90,000 loss. The first pays one third ($30,000) and the second two thirds ($60,000). An equal split would ignore the difference in limits.
"Special physical damage": the CDI outline lists it next to primary and excess coverage without defining it. One reading is the Part D transportation expense benefit that a PAP adds after a theft (see the Part D settlement page).