Underspecification is the phenomenon (sometimes called neutralisation) illustrated by the use of 0 in the Intermediate Tagset. It means that the distinction between the different values of an attribute is not relevant in this instance. One could also say that the particular attribute marked 0 is not applied to the textword under consideration. The possible reasons for this are threefold:
There is room for different viewpoints on whether morphological syncretism should lead to underspecification of values, or whether values, even where they are not morphologically signalled, should be specified on the basis of context. There is also room for difference of opinion about whether the unmarked value of a binary attribute should be applied to the absence of the marked value. (E.g. should we mark all verbs which are not passive in Danish as active? Or should we leave Voice unspecified, except with those verbs for which the passive is an option?)
However, the important point to make here is that underspecification is normally signalled, in a tagset, simply by the absence of any indicator of the attribute. Alternatively, as in the Intermediate Tagset, a 0 is used to make the absence of an attribute explicit.