Physical Damage is what your personal auto calls comprehensive and collision coverage. With trucking there is not a separation between whether it was a collision incident or a comprehensive. It is a claim, and you pay your deductible.
Just like with your personal auto physical damage coverage is only required if you have a loan or lease on the truck and/or trailer. Physical damage covers repairs to your truck and/or trailer in the event you are in an accident and deemed your fault. The premium for this coverage is driven off the stated value of the equipment. The higher the value the higher the premium.
Note: Each year you will want to lower the stated value on the equipment since it has depreciated. If your agent isn't asking each year what the values are then you may be over paying for this coverage and will not get that back.