This document presents a reference framework for classifying software quality models. It proposes analyzing software quality using four "worlds": 1) Subject World, which defines what software quality is, 2) Usage World, which identifies user intentions and goals for quality, 3) System World, which specifies how quality will be represented and measured, and 4) Development World, which contains processes and tools to achieve quality objectives. Each world addresses a fundamental question about quality. The framework characterizes each model based on attributes within these worlds. It aims to provide a comprehensive way to analyze, compare, and develop software quality models.