The drawings by query mechanism that Smart 3D uses requires that all piping isometrics have a target object that is used as a starting point. This means that an isometric is always created for a specific object (for example a pipeline, or spool, or assembly) and extraction rules are used to determine which parts (related to the object) need to be included in the isometric drawing.
However, sometimes you might need to create and manage an isometric drawing for a group of connected parts that do not have such an object as a parent. For example, you might need to control the sheet assignment for large pipelines which do not fit into a single sheet. You might also decide to issue a revision for only one sheet of a pipeline that contains multiple sheets. Although pipelines can extend across module boundaries, isometrics might only need to be created and issued for a subset of the parts of the pipeline that belong to a particular module. On the other hand, a large network of piping across multiple pipelines may need to be stress analyzed and a stress isometric needs to be created that contains parts from several pipelines.
To allow this, Smart 3D supports the creation of isometrics from WBS items. When a pipeline isometric is created, the pipeline exists before the parts that belong to it are modeled. Relationships are established between pipelines and parts (via runs) as a result of normal modeling activity. However this is not the case for WBS items. So WBS isometric creation has additional steps that include creating the WBS item, claiming the parts to a WBS project, and then assigning the parts to the WBS item before the isometric can be created.