If a parachain needs to have assured block inclusion at each Relay Chain block, it must acquire a parachain slot. http://images.google.ml/url?q=https://Xn--L3CA4B8Epb5B.com/category/%E0%B8%84%E0%B8%A5%E0%B8%B4%E0%B8%9B%E0%B8%AB%E0%B8%A5%E0%B8%B8%E0%B8%94/