Sound Transmission Class is derived from other sound metrics like NIC and NIIC. You can read more about NIC and NNIC here. 

The NIC and NNIC don’t attempt to describe how sound gets from one room to another. Does the sound go through cracks or around corners (called flanking paths) or does it go straight through walls or other partitions (called direct transmission)?  By taking care to block flanking paths, the noise reduction between two rooms can reasonably be assumed ...