Whenever your San Francisco courier service does up an invoice for your company, they are going to use a number of different figures to help them find out how much to bill you in the end. One of the most important parts of this is the freight class. Basically, this tells them how hard your item is going to be to ship, and how much investment of space it is going to cost them. They then combine that number with the distance to come up with a starting figure for your quote.
Obviously, space is the one thing that courier companies need to ration the most carefully. No matter how skilled they are at packing a vehicle, a vehicle only has a certain amount of space inside of it. On top of that, transportation laws have limits on how much weight any vehicle can carry. That said, they need to ration their real estate in each shipment that they send out. Part of doing that is charging each customer a rate appropriate to the size and weight of their shipment. That is why both of those numbers are part of how your freight class is arrived at.
The difficulty in transporting your shipment is also something that is definitely used to adjust your freight class. This will only put you into a higher or more expensive freight class. If no adjustments were made, the rate would still be the same based on the size and weight of your shipment. There are many things though which might cause your package to be more difficult to move or transport than similar package, something which will give you a higher freight class and more expensive bill.
There are many things that would apply in this situation. These include things like shipping very valuable goods. That is because they have to be handled very slowly, and can take up more room than necessary because nothing can be piled on them. Anything weighing over 100lbs would also have this applied, as special equipment is needed to load it in and out of the trailer which is carrying it.
Freight classes provide a quick, easy, and standardized way for couriers to simply plug numbers into an equation in order to find a fair amount to bill you. The exact amount will differ from courier to courier of course, even though the basic freight class will be identified in a very similar manner.