Target Challenge (TC)
Your target challenge (TC) represents how hard you are to target in an attack made on you. Essentially the smaller you are, the faster you are, the more obscured you are, and the more aware of your surroundings you are, the harder you are to hit.
The TC is the number you need to get over to hit your opponent with a d20 target roll.
The size factor (SX), is an adjustment to your TC, based on your size, and is the last column of the table shown under the size trait.
Certain items of equipment can also affect your TC