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14.13 Questions 367 the allocation. In this way, allocation is used to direct the system design activity through the model without prematurely deciding how the relationship between model elements will be refined. Of course, this is very dependent on the modeling method. SysML allocations allow the modeler to keep model refinement options open. For example, functional allocations can be refined by designating activities allocated to a block as methods called by operations of the block; this, of course, requires the additional step of creating the operations. Deferring the decision allows the modeler to work at a consistent level of abstraction, and not to get prematurely drawn into modeling details or methodological trade-offs. Even after the model is refined, it is appropriate to retain the allocation relationships, possibly capturing supporting «rationale» in the model to provide a history of how the model was developed. This can be very important information when considering reuse of the model on a different program or product. 14.12 SUMMARY The allocation relationship provides significant flexibility for relating model elements to one another beginning early in the development process. Key concepts for modeling allocations include the following.