(5)To calculate K IRB separately for each pool, institutions calculating K IRB should be allowed to split the pools of qualifying securitised exposures into homogeneous sub-pools. That flexibility is necessary because the composition of the underlying exposures of a securitisation is often heterogeneous. However, each sub-pool should fully comply with those requirements laid down in Article 255(4) of Regulation (EU) No 575/2013 that apply to pools of qualifying securitised assets.