While an architecture defines structure and behavior, it is not concerned with defining all of the structure and all of the behavior. It is only concerned with those elements that are deemed to be significant. Significant elements are those that have a long and lasting effect, such as the major structural elements, those elements associated with essential behavior, and those elements that address significant qualities such as reliability and scalability. In general, the architecture is not concerned with the fine-grained details of these elements. Architectural significance can also be phrased as economical significance, since the primary driver for considering certain elements over others is the cost of creation and cost of change.
Since an architecture focuses on significant elements only, it provides us with a particular perspective of the system under consideration -- the perspective that is most relevant to the architect.8 In this sense, an architecture is an abstraction of the system that helps an architect manage complexity.
It is also worth noting that the set of significant elements is not static and may change over time. As a consequence of requirements being refined, risks identified, executable software built, and lessons learned, the set of significant elements may change. However, the relative stability of the architecture in the face of change is, to some extent, the sign of a good architecture, the sign of a well-executed architecting process, and the sign of a good architect. If the architecture needs to be continually revised due to relatively minor changes, then this is not a good sign. However, if the architecture is relatively stable, then the converse is true.