Modulation is the act of utilizing a certain function to the degree that the corresponding signals manifest visibly in the person's vultology. Every person can modulate all four functions to some minor (or major) degree. As we encounter situations in our environment, we call upon the functions we need in order to navigate the situation; this is modulation. For example, if we are giving a speech in class we will likely modulate Je slightly more from wherever our energetic baseline is, and if we are playing with children we we likely modulate Pe more than our baseline.
Modulation is a temporary activity and does not necessarily reflect a person's development level, unless a certain state of modulation has become the most typical expression of their body across various contexts. Thus, one instance of modulation does not mean there is full function development, but the habitual display of that modulated state does strongly suggest it.