"It is not the strongest of the species that survives,
nor the most intelligent,
but rather the one most responsive to change."
In UBL you have the opportunity to react on changes and given requirements by customizing the existing UBL documents following certain rules.This "Customization Area" of ubl.xml.org allows you to present and discuss your customizations of UBL done or planed with others in order to reach the highest level of interoperability by re-using existing customizations and to avoid uncontrolled growth of potential wrong customizations.
In many small-business environments, standard forms can satisfy business requirements well enough to be used without modification. The existence of standard paper forms such as the UN Layout Key proves this. In these environments, UBL can work right out of the box.
It is true, however, that different industries have different data requirements, and this has led in the past to the proliferation of variants even in such tightly controlled standards as X12, EDIFACT, and RosettaNet.
UBL does not attempt a complete solution to this problem but instead takes an extremely pragmatic approach that should allow satisfactory solutions in the great majority of real-world cases. For example, each UBL 2.0 schema contains an optional extension area in which trading partners may, by agreement, include any data not already covered by the very extensive predefined UBL data structure. Maintaining this extension area and coordinating its use is, of course, the responsibility of the trading partners. But this simple strategy allows nearly unlimited flexibility in individual trading relationships without requiring modification of the standard UBL schemas.
UBL Guidelines for Customization
March 2010 - Turkey - http://lists.oasis-open.org/archives/ubl/201003/msg00014.html