Let's take a look at the format of TAs.

In the right side of the figure are a Tracking Area Identifier (TAI) and a Tracking Area Code (TAC). A TAC is the unique code that each operator assigns to each of their TAs (e.g. TA1=0x0001 for A neighborhood, TA2=0x0002 for B neighborhood, etc.). A TAI consists of a PLMN ID and a TAC. Here, a PLMN ID, a combination of a Mobile Country Code (MCC) and a Mobile Network Code (MNC), is the unique code assigned to each operator in the world. Korea's MCC is 450, and SKT's MNC is 05. So, SKT, a Korean operator, has an MCC of 450 and an MNC of 05. This format of assigning makes a TAI uniquely identified globally.

We will go further and learn about Tracking Area Update (TAU).

As seen above, the LTE network (the MME, to be accurate) has to have updated location information about UEs in idle state to find out in which TA a particular UE is located. For this, the UE notifies the LTE network (MME) of its current location by sending a TAU message (TAU Request message) every time it moves between TAs.  

To explain this process further, a UE obtains a TAI list when it attaches to an LTE network. This list shows the tracking areas where the LTE network believes a UE is located and within which a UE can travel without TAU. In the example shown in the figure above, the TAI list has {TAC1, TAC2}. This means the UE does not have to send a TAU message to the MME as long as it stays in TA1 or TA2, but it has to send one to the MME when it moves to a new TA other than the two (e.g. TA3). The MME is supposed to provide the UE with a new TAI list reflecting the specific details of the UE's move (e.g. new location, moving speed, etc.) for more efficient paging.

One more thing worth mentioning is Periodic TAU, through which a UE in idle state sends a TAU message (TAU Request message) to an MME periodically even when the UE stays within a TA in the TAI list. If a UE in idle state has stayed in one location (or moved within the TAs in the TAI list) and has not notified the MME of its current location, the network cannot tell whether the UE is still in idle state, or is not able to communicate. So, the UE, even when the TA is not changed, sends TAU Request messages to the MME periodically  to announce "it is able to receive data". Otherwise, the network believes the UE is not able to receive data and does not perform paging even when there is data traffic heading to the UE.