The relative ones rely on comparative points of reference: “the lighting in the photo was ‘harsh’” or “the smell of garlic was ‘strong’”. Absolute ones are quantitative units and, of course, more accurate and useful than relative ones, especially in the case of highly precise analyses.
Each signal therefore has a specific unit of measure. Let’s begin with a quantitative one. Sound measurement, for instance, uses the frequency (Hz) or loudness (db) of the sound as the unit of measure. The lumens are one of the quantifiable unit of measure from light.
But an assessment of the taste of garlic is also a unit of measure, albeit a less measurable one than hertz or lumens. The contact of skin with a doorknob is a signal. How that contact ‘feels’ is the unit of measure. The smell of burning wood is a signal with pungency as one of its unit of measure.
A signal is every independent connection we have with the world that surrounds us. This connection is made by one of our five senses. We see with our eyes, hear with our ears, feel with our skin, taste with our tongue and smell with our nose. As we shall see, understanding these links, especially their limits, is one requisite to better communications.