A library unit is a separately compiled program unit, and is always a package, subprogram, or generic unit. A root library unit, together with its children and grandchildren and so on, form a subsystem. An object is either a constant or a variable. An object contains a value. A subcomponent of an object is an object.
Packages are program units that allow the specification of groups of logically related entities.
A partition is a part of a program. Each partition consists of a set of library units. Each partition may run in a separate address space, possibly on a separate computer. A program may contain just one partition. A distributed program typically contains multiple partitions, which can execute concurrently.