Categorizing things is a method people use to survive. Searching the internet on this topic yields many interesting studies and theories about why we do it. Categorization helps us to identify a situation quickly, categorize it and act accordingly. We do it all the time. This approach rings true not only of individuals, but also with communities, businesses and industry sectors.
Defining categories for potency of pharmaceutical compounds has been invaluable to our industry. It allows R&D to evaluate and define the potency of new compounds so they may be handled safely. Architects and engineers are then able to seek the right approach for facility design with appropriate material and personnel flows as well as process design with suitable engineering controls for containment. Equipment vendors offer containment solutions, most often marketed for the common industry operator exposure level (OEL) bands: less than 1 μg/m3, 1-10 μg/m3, 11-100 and so on.