The purpose of specialized subsets of the full PDF specification is to remove those functions that are not needed or can be problematic for specific purposes and to require some usage of functions that are only optional (not mandatory) in the full PDF specification.
制定完整PDF规格的子集的目的是移除那些不需要或会对特定用途造成问题以及一些要求的功能的使用在完整PDF规格中仅仅是可选的（不是强制性的）功能。