Barney stated that for resources to hold potential as sources of sustainable competitive advantage, they should be valuable, rare and imperfectly imitable.[75] A key insight arising from the resource-based view is that not all resources are of equal importance nor possess the potential to become a source of sustainable competitive advantage.[73] The sustainability of any competitive advantage depends on the extent to which resources can be imitated or substituted.[6] Barney and others point out that understanding the causal relationship between the sources of advantage and successful strategies can be very difficult in practice.[75] Barney uses the term "causally ambiguous" which he describes as a situation when "the link between the resources controlled by the firm and the firm's sustained competitive advantage is not understood or understood only very imperfectly." Thus, a great deal of managerial effort must be invested in identifying, understanding and classifying core competencies. In addition, management must invest in organisational learning to develop and maintain key resources and competencies. A Beginner's Guide to Branding Your Business