There is such a wide variety in the things that people can ship with courier companies that most shipments have very little in common with each other. If one company is shipping a tube of drafting plans via bike messenger and another is shipping three truckloads of auto parts across the country it would seem like they would have almost nothing in common, but there is a single common denominator. That is that every shipment with any Hollywood messenger, delivery, or trucking company is going to be accompanied with a reference number. This is a number that you will be given on all of your paperwork when you place the order for your shipment, and it is very important that you keep track of this number, as it is the most important piece of information you're going to have regarding your shipment. All of the relevant data in your shipping file at the courier company will be tied to this number, so it is important that you keep it to hand.
The reason it is so important for you to keep your reference number is because if you ever have any questions about your shipment, the first question the company is going to ask you is "What is your reference number?" If you have any concerns about your shipment, they will use this number to look up all the information regarding it, and this even applies after the shipment is completed, for example if there was any kind of a billing dispute. Secondly, if you want to track your package in any way you are going to need your reference number. The two primary kinds of tracking systems that messenger services have in place, via the phone or the internet, both require a tracking number so that they can look up your shipment easily and let you know where it currently is.
Reference numbers are often called tracking numbers by courier companies, so don't be confused if the company that you use uses one term or the other, they mean essentially the same thing. The reasons some companies call them tracking numbers is simply because that is what they are most commonly used for. The other uses of reference numbers such as looking up past relevant details on the shipment don't come up that often, but almost everyone is interested in tracking their shipment to make sure that it is progressing safely towards its destination.