Celluar Network including LTE has very complicated structure and it is made up of may different layers. In addition, the network is communicating with many different user (UEs) at the same time. So if you pick any one point along the network architecture and observe the traffic following in and out of the point, it is highly likely that many traffics for multiple users are following through the point. In this situation, how each of the network components can figure out which traffic is for which UE. Also when the traffic gets transmitted onto the air, each UE is recieving all the physical layer data not only for itself but also for other UE. In this case how UE can figure out which data it has to decode and which data it has to discard.
To solve this issues, we use specific IDs. But in many cases different network components uses different types of IDs even for the same UE and same data. For example, even for the same data and the same UE, the type of IDs between UE and eNB and type of IDs on X2, S1, S5 interfaces may be different. So it will be very difficult and very confusing to follow through a single traffic from a UE to final destination. Also, when you are discussing with other people about the traffic and IDs, you may easily confuse others (sometimes confuse yourself :)) if you don't specify exact point along the overall data path and explicitely mention on your intention of the discussion.
Before you read through this page, I recommend you to go through the diagram at this link LTE-Identifier and get a big picture on what kind of ID is used at each point on overall data path. In this page, I will keep adding specific details of each IDs and examples of real signaling message so that you can use this page as a dictionary of IDs. (This is very beginning, so please don't expect the complete dictionary for now :))
Note : Following table is based on the document from MNC Group ( LTE-Identifier )