One of the important goals in revising the existing Guidelines and developing P5 had been the simplification of the customization process which was difficult to understand and tedious to implement in previous versions. The TEI Council accordingly spent considerable time in designing and implementing the new class-based modular infrastructure building blocks. In this new system, customization is not an afterthought but is required for virtually every use of the TEI.
(Wittern et al. 2009, 293)