"Originally UML was intended to serve as a . But a specification is primarily intended to describe properties of systems that the system developers want to be valid, but to leave open other properties that are not clear already. Today this is partly achieved by having a semantics that is rather vague (and here we mean imprecise as opposed to not detailed). However, this is not an advantage, as the developer cannot fix this kind of impreciseness within UML, but can adapt the individual interpretation only. Furthermore, to get complete (and therefore executable) UML descriptions, often certain details have to be specified, which the developer does not yet know or wants to leave open to a later phase of development or even implementation. It is an intrinsic problem of executable languages that this kind of over-specification frequently occurs. Instead it would be of some help to have flexible concepts of under-specification to postpone detail decisions to situations, where the decisions can and must be made."