The time Ross, Alexander and Julia spent training in the accelerated time field not only encompassed grasping their given laws. Due to Alexander's nature as a Veilwalker and his speed of growth, he did not have the time to explore the inner capabilities of each power stage.
But he had all the time he needed to explore this with Ross and Julia. Through their trials and training, they were able to break down what each stage had to offer. The Semi-Immortal stage was more than just the beginning of grasping laws to a threshold of 25%.
This stage elevated the evolution and life force of the being to higher levels, effectively rendering them nearly unbreakable entities.
Semi-Immortals were capable of freely controlling their bodies and freezing their ageing process up to a certain stage, so long as their life force was not close to its end.
