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The modeling concepts of UML are grouped into language units. A language unit consists of a collection of tightly-coupled
modeling concepts that provide users with the power to represent aspects of the system under study according to a particular
paradigm or formalism. For example, the State Machines language unit enables modelers to specify discrete event-driven behavior
using a variant of the well-known statecharts formalism, while the Activities language unit provides for modeling behavior
based on a workflow-like paradigm. From the user’s perspective, this partitioning of UML means that they need only be concerned
with those parts of the language that they consider necessary for their models. If those needs change over time, further language
units can be added to the user’s repertoire as required. Hence, a UML user does not have to know the full language to use
it effectively.
In addition, most language units are partitioned into multiple increments, each adding more modeling capabilities to the previous
ones. This fine-grained decomposition of UML serves to make the language easier to learn and use, but the individual segments
within this structure do not represent separate compliance points. The latter strategy would lead to an excess of compliance
points and result to the interoperability problems described above. Nevertheless, the groupings provided by language units
and their increments do serve to simplify the definition of UML compliance as explained below.