The IFC model has such a wide scope that no one application is going to implement the whole model. Defining views ( =subsets) of the model is the way to ensure that applications that implement the part of the IFC model they are interested in do it in an consistent manner. It is also easier to study and understand a view of the model than to deal with the whole model.
The BLIS project approaches the question of views with 'concept blocks'. Concept blocks define a simple functionality that is enabled by a small part of the IFC model. A view is then formed by linking together concept blocks that are needed for a specific purpose, like Arch. design >> Quantities take off / cost estimating.
(Related document : IFCR2_Concept_block_approach_000524_jh.pdf)
Online version of the BLIS view definitions