All objects are in one of four stages: Virtually all types, styles, methods, or objects of meditation are included in one of these four stages, levels, or categories (1.17). The specific objects within those four stages are discussed in later sutras. (See also the articles, Five Stages of Meditation and Types Versus Stages of Meditation.)
Objectless concentration: The four stages (above) all have an object to which attention is directed (samprajnata). Beyond these four is objectless concentration (1.18), where all four categories of objects have been released from attention (asamprajnata).