Once a bank adopts an IRB approach for part of its holdings, it is expected to extend it across the entire banking group. The CBB recognises however, that, for many banks, it may not be practicable for various reasons to implement the IRB approach across all material asset classes and business units at the same time. Furthermore, once on IRB, data limitations may mean that banks can meet the standards for the use of own estimates of LGD and EAD for some but not all of their business units at the same time. CBB will expect banks to define their business units in line with asset classes given in this chapter, however banks can apply to CBB for exemption from this rule.