Understanding absolute occurrences

You can use absolute occurrences to manage data that is unique to a specific use of an item within a product structure. If you have ownership of and write access to a particular BOM view revision, you can define a subset of BOM line properties that other users can edit in the context of the BVR. You can also allow users to attach forms or datasets in context, by configuring business rules that define the types of primary and secondary objects for the relationships. When a user enters in-context values or attachments, Teamcenter overrides certain structural data associated with a relative occurrence in the context of a specific higher level assembly. Data you can override includes: Occurrence notes Occurrence type Quantity Absolute transform Find number Variant condition Attachment, for example, a JT file for a different rendering Component item for a replacement A BOM line represents an absolute occurrence with respect to the current top line in the product structure. You can set occurrence attribute values on such a BOM line, without those values appearing in the product structure everywhere the BOM line's parent appears. There are cases where a user needs to store values specific to the BOM line with respect to the top line or some intermediate assembly between the BOM line's parent and the top line. Absolute occurrences are always created in the context of a selected top line; the same absolute occurrence may not be in context for another top line. The following figure shows an example where two vehicles are built on the same chassis. The chassis contains two occurrences of a suspension system, one for the front of the vehicle and one for the rear; these occurrences are positioned relative to the chassis by transforms T3 and T4, respectively. The suspension assembly contains two occurrences of a wheel assembly, one for the left side of the suspension system and the other for the right side of the suspension system. The wheel assembly occurrences are positioned relative to the suspension assembly by transforms T1 and T2, respectively. The wheel assembly is composed of a wheel, tire, and valve stem. The occurrence of the tire is annotated with a recommended tire pressure of 30 PSI. Sample product structure with relative occurrences only Relative occurrences correlate directly to structure relationships. If the structure relationship has a quantity associated with it, the number of relative occurrences associated with the structure relation equals the quantity. In the previous figure, when you display the product structure for vehicle Y, every occurrence of the tire shows a recommended tire pressure of 30 PSI; that is, the value set on the occurrence of the tire in the wheel assembly is propagated to all BOM lines for the tire. Positioning information is derived in the same way. The position of the wheel assembly in a vehicle is derived from transformation values specified on the (relative) occurrences in the product structure, for example, the position of the front left wheel is derived by multiplying T1 and T3. Consider a case when the position of the front wheels must change so they toe-in by 0.5 degrees or the case where the recommended tire pressure for the rear-tires in vehicle X must change to 33 PSI. One way to accomplish this is to modify the structure, but in most situations, this approach is not practical. Instead, you should make context-specific modifications to BOM line properties without changing the product structure. To permit such context-specific modifications, you can create absolute occurrences. Creating absolute occurrences allows you to effectively flatten the representation of a structure while knowing how to map back to the actual product structure. The following figure illustrates this concept, expanding on the preceding example. Sample product structure with absolute occurrences The dashed lines in the figure represent absolute occurrences. In this case, the absolute occurrences specify or override values in a specific context. In the example, they override positioning information and recommended tire pressure. The context of the override and the value is shown in the label on each dashed line. Specifically, in the context of the vehicle-X, the front tires have a recommended tire pressure of 30 PSI; this is derived from the relative occurrence of the tire in the wheel assembly. However, the two rear tires have a recommended tire pressure of 33 PSI; this is explicitly set on the absolute occurrence in the context of the vehicle. The value of 33 PSI on the absolute occurrence overrides the value of 30 PSI that appears on the relative occurrence of the tire in the wheel assembly. Vehicle-Y, however, has a recommended tire pressure of 30 PSI for all four tires. Positioning information is derived in a similar way. The values specified on the absolute occurrence override the values that are otherwise derived by catenation of the transforms from the relative occurrences. That is, in the second figure, T5 overrides the value that would otherwise be derived from the multiplication of T3 and T1. Because this example specifies the transform overrides in the context of the chassis, both vehicle-X and vehicle-Y use T5. If you release an intermediate BOM line that represents a subassembly, the line is locked and you do not have write access to it. However, you can still make changes to its absolute occurrences in the context of a parent assembly whose BOM view revision is also released, for example, to attach a dataset with associated JT files.