The UNL is supposed to be a universal language, but the adjective "universal" must be understood, in the UNL framework, in terms of "semantic accessibility", i.e., as the capability of being used and understood by all. This is the use of "universal" that one may observe in "universal adapter", "universal screwdriver" and "universal remote control", for instance. This does not mean, obviously, that UNL is a sort of super-language adapted to various purposes, forms and operations; it only means that the UNL is not bound to a particular language, and may be used to represent information in a way that it can be processed independently of the source language or the target language.
The idea of "universality" in UNL is strictly related to the notions of "explicitness", "self-sufficiency" or "language-independence". In order to be universal, the UNL has to be neutral, because a constructed language cannot be semantically accessible if it is bound to any particular language. In the UNL framework, this semantic neutrality is associated to the idea of closed system, as defined in structural linguistics, i.e., as a system of interdependent entities, where the value of a sign is determined inside the very system, by all the other signs to which it is related. In order to be self-sufficient, the UNL must work only through relations of difference, either syntagmatic or paradigmatic, which place signs in opposition to one another, delimiting their meaning and possible range of use. This system is the UNL Knowledge Base, which is the content plane of the UNL. The value of a UW, for instance, is not defined positively, by reference to an external entity (such as a word of English), but by its position in the UNL KB, i.e., by the set of relations departing from or coming to this UW. This relational, negative, structuralist definition assigns autonomy (and hence universality) to the UW.
It should be stressed, however, that this semantic neutrality is still a desideratum. The current UNL is still language-biased and depends intensively on implicit knowledge. But this is an ongoing initiative, and we do hope that eventually we will come up with a formalism that will be progressively more tailored to represent information in a less-marked way.