Each race has modifiers for developing their abilities. A lower the number means that it costs less to raise that ability with training, a higher number means it is more difficult. In other words, a -2 means it's very easy and takes less TDPs to train, while a +3 means that it's very difficult to train that attribute. Example - if you are human (racial modifier of 0), with an 10 in strength, it would take 30 TDPs to raise your ability to 11.