The 8 Principles are general characteristics taken from several commonly used diagnostic models. Together they're an analytical tool for discovering what pathologies are present or revealing information about a sign or symptom that will allow more specific classification of it. This helps narrow the field of possibilities when rendering a diagnosis from a huge pile of undifferentiated information. For any phenomenon there are four distinctions to be made.
A man complains of waking up at night to urinate. He also feels somewhat cold most of the time, has cloudy urine, and feels fatigued most of the time.
He is Qi deficient which is a Yang pathology. The cloudiness in his urine is an Excess. None of the External Qi are present so this is an Internal pathology. He literally feels cold. His * Principles diagnosis is therefore Internal Deficiency of Yang with Cold.